Before you start…

This is an advanced worksheet, which assumes you have completed the Absolute Beginners’ Guide to R course, the Research Methods in Practice (Quantitative section) course, and the Intermediate Guide to R course.

Contents

Overview

This worksheet describes a full analysis pipeline for an undergraduate student dissertation on the Perruchet Effect. The experiment was basically identical to the experiment described by McAndrew et al. (2012), except that the electric shock was replaced by a loud unpleasant noise (a metal garden fork scraping across slate). For details of the procedure, see the McAndrew paper, but here’s a very brief summary:

This was a conditioning experiment. A shape (brown cylinder, known as a conditioned stimulus, or ‘CS’) appears on the screen for 8 seconds. On 50% of occasions, this is then followed by the unpleasant noise (the unconditioned stimulus or ‘US’). The participant’s reaction to each CS is measured in two ways:

  1. While the CS is on the screen, they are asked to rate their expectancy of the US on each trial, using a numerical scale. Higher ratings indicate greater conscious expectation of the unpleasant noise.

  2. Throughout the experiment, their Galvanic Skin Response, a.k.a. electrodermal activity is measured. This is a measure of physiological arousal, greater numbers mean more arousal. Higher arousal is interpreted as greater unconscious expectancy of the unpleasant noise.

So, we have two dependent variables - expectancy ratings, and GSR. There is also one independent variable - “run length”. This relates to how many trials in a row you have experienced the unpleasant noise. For example, a “+3” trial means you have experienced the noise three times in a row; a “-2” trial means you have experienced the absence of the noise two trials in a row. The run lengths in this experiment range from -3 to +3.

There are two predictions. The first prediction is that the GSR will increase with run length, because the unconscious association between the shape and the noise will grow with repeated pairings. The second prediction is that the conscious expectancies will do the opposite - they will decrease with run length. This is known as Gamblers’ Fallacy - in this case, the belief that, even though the sequence is random, a run of unpleasant noises means the next trial is more likely to be silent. This result is often taken to be one of the strongest pieces of evidence for a dual-process theory of learning, i.e. that we have unconscious and conscious learning processes that act somewhat independently of each other. This is because it can be hard to see how a single learning process could believe two things at the same time e.g. that on the +3 trials, it was simultaneously most and least likely that an unpleasant noise would occur.

Getting data from a public repository

A basic principle of open science is that you publicly share your raw data (in anonymized form). Many psychologists, including the ones who ran this study, choose the OSF website to do this. So, the first thing you need to do is download the data from OSF. Of course, there’s an R package for that (osfr).

So, first, create a new project in R.

Next, create a new, empty R script and save it as perruchet.R. Put all the commands into this worksheet, and run them from there. Save your script regularly.

Now, download data from OSF into your R project using the following commands:

## Load data from OSF (commands only need to be run once)
library(osfr)
proj  <- osf_retrieve_node("r7axw")
files  <- osf_ls_files(proj)
osf_download(files, conflicts = "skip")

Explanation of commands

The first line is a comment. These are ignored by R, but make your script easier for humans to read. In this case, the comment reminds the human reader that you only need to download the data from OSF once.

The second line loads the osfr package.

The third line asks the OSF website for the information it needs to download the data. To do this, we need the last part of the web address. This repository is at https://osf.io/r7axw/, so we need to use r7axw.

The fourth line gets a list of the files in the repository, much as we did for local files in Preprocessing data from experiments.

The final file actually downloads those files to your R project. In this case, you’ll find that you now have a directory called data, which contains those files. The component conflicts = "skip" means that, if these files are already in your project, you should not overwrite them, but leave them as they are.

Looking at the downloaded files

You can ignore the file MAIN.csv, which we’re not going to use. The first file you should open is codebook.md. A codebook is a file that explains what each of the columns in a data files means. Read the codebook and, while you’re reading through, look at the data file it is talking about, DATA.csv. It is this file on which all the following analysis is based.

Loading data

As with any analysis, the first step is to load the data into R:

## Load packages, data. Rename data frame columns
library(tidyverse)
raw <- read_csv("data/DATA.csv")
colnames(raw) <- c("subj", "time", "key", "value")

Explanation of commands

The second and third lines should be familiar from previous worksheets; we nearly always need the tidyverse package, and we use the read_csv command to load the data. The data/ part of the filename data/DATA.csv says that the file DATA.csv is to be found in the directory (folder) called data.

The final line renames the columns, as covered in the Preprocessing data from experiments worksheet. I didn’t like the names of the columns very much so I changed them to something more comprehensible and quicker to type, using the colnames command.

De-duplicating data

We’re now going to get the data into a state where we can meaningfully analyse it:

## De-duplicate data; remove pilot study
raw <- distinct(raw)
raw  <- raw %>% filter(subj > 161)

Explanation of commands

The second line, raw <- distinct(raw) is a de-duplication command. It’s a very handy command for tidying up data in cases where people have made the grave error of trying to use Excel for serious research. What happened in this case was that the experiment was run by three people over many different days. They then tried to stitch all the data files together by copying-and-pasting them into one big file in Excel. This would have been easy using R, as covered in Preprocessing data from experiments but, unfortunately, no one had taught them how to use R. Predictably, they could not do this combination of files in Excel without error (few people could), and they ended up copying some of the data twice. In this case, it’s easy to detect and fix in R, because no two rows of this data can be identical - the subject number or the time is always going to be different. So, we can remove duplicates by telling R to give us just one copy of each of the different rows in the data set. The command distinct does that for us.

The final line, raw <- raw %>% filter(subj > 161) should be familiar from previous worksheets; we are keeping only those participants whose subject number is greater than 161. This is because, in this study, we first ran a short pilot experiment, looked at the data, and made a few changes to the experiment. Here, we only wanted to analyze the main study, so we removed the pilot participants.

Excluding participants

Participants in this experiment were told to rate their expectancy of the US during the CS, on every trial. However, they only had 8 seconds to do this, and the experimenters noticed that sometimes people failed to make a rating. The experimental apparatus records this as a rating of zero.

Participants who miss a lot of ratings aren’t really following the instructions, making their data hard to interpret. In such cases, we normally exclude those participants from further analysis. First, we need to find out how widespread this problem is. One way to do this is to calculate the number of zero ratings each participant makes:

## Number of expectancy ratings, by participant
inspect  <- raw %>% filter(key == "ER") %>% filter(value == 0)
table(inspect$subj)

163 164 166 176 184 188 191 196 199 202 223 233 235 236 239 246 
  1  19  46  16   2   3   2   1  32   3   3   1   3   9  34   1 

Explanation of commands

The second line filters to those rows of the raw data that contain expectancy ratings (key == "ER", see codebook.md) and then to cases where that rating is zero (value == 0). This is then stored in the data frame inspect. One could look at this manually, but it’s more efficient and less error prone to get R to tell us how many times each participant has a zero rating. This is what the final line, table(inspect$subj) does - it counts the number of times each subject number occurs in inspect.

Explanation of output

The output shows the number of trials on which each participant failed to give an expectancy rating. If a participant never made a zero rating, their participant number does not appear on the table. We can see that sixteen participants missed at least one rating, and that six participants missed more than 3 (one participant never made a rating, missing all 46 trials of the experiment!).

Removing participants from the dataset

In this case, we decided to exclude all participants who missed more than three ratings:

## Exclude participants
exclude  <- c(164, 166, 176, 199, 236, 239)
raw  <- raw %>% filter(!(subj %in% exclude))

Explanation of commands

The second line creates a list of the participant numbers we wish to exclude. In the second line, we remove those participants from the raw data frame. The filter command will be familiar from previous worksheets. The command subj %in% exclude means ’any participant whose subject number is in our list exclude. The use of !() in the filter statement means not. So, filter(!(subj %in% exclude)) means keep the participants whose subject number is not in the exclude list.

Note

There are still a few trials where the participant makes no expectancy rating, and the computer records this as an expectancy of zero. There’s a good case to be made for recoding the rating on those trials as either NA (meaning ‘missing data’) or 3, meaning “participant is unsure whether the noise will occur”. For brevity, we don’t make these changes here, but once you’ve gone through the whole worksheet, perhaps try to work out how one might do this.

Graphing GSR

The GSR data from this experiment is very rich, with a skin conductance measure taken from each participant approximately twenty times a second throughout the whole 30-40 minutes of the experiment. To ‘get a feel’ for these data, and also to check that the equipment was working properly, we need to plot that GSR as a graph, with time on the x-axis and GSR on the y-axis. We’ll need to do this one participant at a time, otherwise the graph will be really hard to read:

## GSR plot
dat  <- raw %>% filter(subj == 168)
gsr <- dat %>% filter(key == "GS")
gplot <- gsr %>% ggplot(aes(x=time, y=value)) + geom_line()
gplot

Explanation of commands

The second line should be familiar, we’ve filtered the data to include just one participant (168).

The third line is also a simple filter operation - we’re keeping just those rows where the key column says GS. From codebook.md, we can see that this means the rows of the data file that contain GSR readings.

The fourth line should also be pretty familiar, we’re just doing a standard line graph, as we previously covered in, for example, the Understanding Interactions worksheet. In this case, we save the line plot in an object called gplot, and then show the graph by typing gplot as the last command. We do it this way so we can add other things to the graph later.

Explanation of output

We can see the GSR going up and down over time. Why is it doing that? Our assumption is that this is people reacting to the CS (cylinder), which is sometimes followed by the US (noise). We can check this by adding a vertical line to the graph for the time each CS occurs.

Adding CS onsets to our graph

We can add CS onset times like this:

## Add CS onset times to graph
cs <- dat %>% filter(key == "CO")
gplot <- gplot + geom_vline(data=cs, mapping=aes(xintercept=time),
                            colour="blue")
gplot

Explanation of commands

The second line should be familiar - we are selecting those rows of the data that contain the key CO, which codebook.md tells us are the timings of the CS onsets.

We’ve also come across geom_vline before, as part of the Putting R to work worksheet - it just plots a vertical line at a place we ask it to. The new part of this command is data=cs, mapping=aes(xintercept=time). Previously, we’ve just told R manually where we want the line (e.g. xintercept = 4.35). Now, we’re telling it that we want a bunch of lines, that the data for the position of those lines is to be found in cs data frame, and that the time column of that data frame should be used to find them.

Here, we’re adding the vertical lines to the plot we’ve already made gplot <- gplot + geom_vline..., and then showing the updated graph, gplot.

Explanation of output

There appears to be a missing blue line in the middle. Actually, there isn’t, it’s just that the participant was given a break half way through the experiment, so there wasn’t a CS for a while. What we can also see, however, is that the GSR seems to go up each time there’s a CS, and then go back down again. Sometimes that’s quite a big rise, and sometimes, less big. If the experiment worked out as we expected, then the size of the GSR reaction will depend on the run length. We’ll look at this later on, but first we need to do some preprocessing.

Baseline correction

One problem with GSR data (and much other neuroscience data, e.g. EEG, fMRI) is that it’s both highly variable (“noisy”), and changes systematically over time (“drift”). By ‘noisy’, we mean that, for a given event in the world, the GSR response to that event varies quite a lot, even if the event itself is objectively identical. In order to deal with this noise, we average the participant’s GSR over several trials, and so get a sense of what their typical response is.

By ‘drift’, we mean that these differences across trials are not random, they are affected by systematic changes in the participant over time. For example, as they hear more and more unpleasant noises, perhaps the noise becomes less unpleasant to them (“habituation”), and so their GSR reaction late in the experiment might be smaller than their GSR reaction early in the experiment. Another example – once someone’s GSR goes up, it can take quite a lot of time to go back down again (“return to baseline”). In this experiment, we waited a long time between trials, the inter-trial interval was around 30 seconds. However, this is often not enough, and we find that sometimes their GSR has not returned to baseline by the time the next CS appears.

The combination of noise and drift means that, not only do we need to average across a lot of trials, we also need to try and make those trials more comparable to each other before we average them. The process we use to do that here is baseline correction. This is simply a subtraction. We calculate the average GSR response in the 500 ms before the CS arrives (the baseline), and then we subtract that baseline from all the GSR readings while the CS is present.

Let’s illustrate this with a single trial from participant 168. For no particular reason, let’s take trial 10:

## Plot a single trial
onset  <- cs$time[10]
full <- gsr %>% filter(time >= onset - 500 & time <= onset + 8000)
full %>% ggplot(aes(x=time, y=value)) + geom_line()

Explanation of commands

Line 2 - In order to find a single trial, we need to know at what time the CS began on that trial. We already have those CS onset times from doing the last graph, so we just ask R for the 10th row of the time column of that cs data frame - cs$time[10] - and record it as onset (with onset <-)

Line 3 - Now we know where the CS starts, we need to take all the GSR readings from 500 ms before that (our baseline), up to 8000 ms after that (when the CS ends). We use the now-familiar filter command to do this.

Line 4 - Just a plain old line graph.

Explanation of output

What we see in the graph is just a ‘zoomed in’ part of the previous graph - for the 10th trial, 500 ms before the CS onset to 8000ms after the CS onset. The x-axis has large numbers because time is counted from the start of the experiment, in milliseconds. The line looks a bit ‘jagged’ because the equipment is has limited accuracy in measuring GSR.

Applying baseline correction

Now, we need to do two things.

First, we need to apply the baseline correction to these data. In other words, we work out the average GSR in the 500ms before the CS arrives; we then subtract that average from every data point, so the first 500 ms of the graph will always be around zero.

Second, we’re going to need to average all these trials, so we need the x-axis to be the same for all of them. To do this, we subtract the onset time from all the values on the x-axis, meaning we end up with data that goes from -500 ms to +8000 ms.

We can do all this in four lines of code, plus one to plot the new graph:

## Basline correct that trial, and replot
precs  <- gsr %>% filter(time >= onset - 500 & time < onset)
correct  <- mean(precs$value)
full <- full %>% mutate(cval = value - correct)
full <- full %>% mutate(rtime = time - onset)
full %>% ggplot(aes(x=rtime, y=cval)) + geom_line()

Explanation of commands

Line 2 - Use filter to select the GSR values for the baseline period i.e. 500ms before the CS onset until the CS onset, and put them in a dataframe called precs.

Line 3 - Calculate the mean of the GSR values in this pre-CS period (precs$value) and store it in correct.

Line 4 - Using the command mutate, we create a new column in the full data frame called cval (for “corrected value”). This column contains the numbers in the value column, minus the baseline correction we’ve just calculated (correct). The name mutate is perhaps a bit odd, but “mutate” is just another word for “change” (over and above it’s other biological, and sci-fi connotations…). So, mutate here just means create a new column by changing an existing one.

Line 5 - Using mutate again, we create another column called rtime (for “relative time”). This is just the actual time, minus the time the CS arrived. So, the CS arrival time will always be zero.

Line 6 - Just another line graph

Explanation of output

We get another graph, which at first sight looks exactly like the last one. The thing to notice is that the x-axis and y-axis now have different numbers - the CS onset is at 0 ms rather than 318000 ms, and the GSR line starts at 0 rather than around 2.5.

Functions and loops

In principle, we could do this baseline correction for each trial of each participant in the same way as the above example. However, this would be really tedious, because there are about 50 participants and 50 trials per participant, so about 2,500 trials in total!

When we want R to do the same thing many times, we can use functions and loops to achieve this. We’ll look at functions first.

Defining a function

Pretty much every command you run in R is a function that someone has written. For example, mean(precs$value) uses the function mean(), which is built in to R. The command read_csv is a function added to R by loading the tidyverse library. You can also write your own functions. Here’s a function that contains all the commands we used to baseline-correct one trial of participant 168:

## Define function for baseline correction
base.correct  <- function(trial) {
    onset  <- cs$time[trial]
    full <- gsr %>% filter(time >= onset - 500 & time <= onset + 8000)
    precs  <- gsr %>% filter(time >= onset - 500 & time < onset)
    correct  <- mean(precs$value)
    full <- full %>% mutate(cval = value - correct)
    full <- full %>% mutate(rtime = time - onset)
    full  <- full %>% add_column(trial = trial, run = cs$value[trial])
    return(full)
}

Ignoring the first and the last two lines for a moment, notice that these commands are almost identical to the ones we just used. The only difference is that we’ve changed cs$time[10] to cs$time[trial]. So, instead of always looking at trial 10, we look at whatever trial is specified by trial, and that is set by calling the function. A function doesn’t do anything until we ask it to, by ‘calling’ it. So, for example, we can now look at trial 9 by calling the base.correct function like this:

eg  <- base.correct(9)
eg %>% ggplot(aes(x=rtime, y=cval)) + geom_line()

The first and last two lines of the base.correct code are what makes it a function. The first line tells R to create a function called base.correct, and that this function takes an input called trial. The { indicates the start of the commands in the function. The } in the last line of code indicates the end of the set of commands in the function. The penultimate line, return(full), means that the function returns the full data frame it has calculated.

So, when we type eg <- base.correct(9), the function we have written does the baseline correction for trial 9, and returns the answers. We put these answers into eg (using eg <-), and then plot the graph.

There’s one line of the function we have not yet discussed:

full <- full %>% add_column(trial = trial, run = cs$value[trial])

The key aspect of this command is add_column which, perhaps unsurprisingly, adds new columns to a data frame. In this case, we are adding two columns to the right hand side of the data frame. These are going to be useful later. The first column contains the trial number, and the second column contains the run length (-3 to +3, see description of experiment). The run length is recorded along with the CS onset time data, in the value column (see codebook.md) so we read the run length for the current trial using cs$value[trial].

Using a loop

We can now use a loop to run this function for every trial of one participant. We’ve used loops in a similar way before, to combine data files from multiple participants using bind_rows, see the preprocessing data from experiments worksheet.

## Use a loop to baseline correct all 46 trials
onep  <- NULL
for(trial in 1:46) {
    onep  <- bind_rows(onep, base.correct(trial))
}

Explanation of command

The loop runs base.correct(trial) for each of the 46 trials the participant did, and adds them all to one big data frame called onep. In more detail:

onep <- NULL - Set up an empty (NULL) dataframe to put our results in.

for(trial in 1:46) { - 1:46 means the whole numbers from 1 to 46. So, the first time we go round this loop, trial is 1, the second time it is 2, and so on. When we get to 46, the loop runs for the last time.

onep <- bind_rows(onep, ...) - Bind (attach) some new rows to onep :

base.correct(trial) - The trials to bind are those that come out of base.correct(trial).

Trial-level summary

Let’s take a look at what we’ve generated:

onep
# A tibble: 7,819 x 8
    subj  time key   value    cval rtime trial   run
   <dbl> <dbl> <chr> <dbl>   <dbl> <dbl> <int> <dbl>
 1   168   551 GS     2.37 -0.0839  -451     1     0
 2   168   602 GS     2.37 -0.0839  -400     1     0
 3   168   651 GS     2.46  0.0111  -351     1     0
 4   168   702 GS     2.46  0.0111  -300     1     0
 5   168   752 GS     2.48  0.0291  -250     1     0
 6   168   801 GS     2.48  0.0291  -201     1     0
 7   168   851 GS     2.48  0.0291  -151     1     0
 8   168   902 GS     2.48  0.0291  -100     1     0
 9   168   952 GS     2.48  0.0291   -50     1     0
10   168  1002 GS     2.54  0.0941     0     1     0
# … with 7,809 more rows

The dataframe onep contains all the GSR data during the CS presentations(and the baseline) for one participant, including columns that record the trial each reading comes from, and the run length of that trial (see experiment description). Each trial is 8.5 seconds of readings at about 20 readings a second and 46 trials, giving us about 7820 readings in total for this participant.

For the purposes of analysis, we need to summarize this data down to just one number per trial, representing how large the GSR was on that trial. There’s a few ways one could do this; here we’re just going to take the average GSR for trial:

## Calculate mean GSR for each trial
gsr.sum  <- onep %>%
    filter(rtime >= 0) %>%
    group_by(trial, run) %>%
    summarise(cval = mean(cval))
gsr.sum
# A tibble: 46 x 3
# Groups:   trial [46]
   trial   run    cval
   <int> <dbl>   <dbl>
 1     1     0  0.361 
 2     2    -1 -0.109 
 3     3     1  0.181 
 4     4     0  0.125 
 5     5    -2  0.0404
 6     6     1  0.0714
 7     7     0 -0.252 
 8     8     0  0.644 
 9     9    -3  0.427 
10    10     0  0.874 
# … with 36 more rows

Explanation of command

Line 3 - We remove all GSR recordings from the baseline period (where rtime is less than zero)

Lines 4-5 - We group the data by trial and by run length, and then summarise this group data by taking the mean corrected value, cval.

Explanation of output

The output shows the contents of gsr.sum. Each row is one trial. For each trial, we have the run length, and the mean GSR response cval. Later on, we’ll analyze these data - but first, we have to finish the preprocesssing.

Adding a second dependent variable

The data frame onep contains a GSR for each trial, but there is also an expectancy rating for each trial. It will make it easier to look at these data if we also include these expectancy ratings into onep. So, using the following commands, we first create a new data frame with the expectancy ratings:

## Extract expectancy ratings
expect <- dat %>% filter(key == "ER") %>% add_column(trial = 1:46) %>% select(subj, trial, value)
expect
# A tibble: 46 x 3
    subj trial value
   <dbl> <int> <dbl>
 1   168     1     2
 2   168     2     3
 3   168     3     3
 4   168     4     2
 5   168     5     4
 6   168     6     4
 7   168     7     3
 8   168     8     3
 9   168     9     4
10   168    10     4
# … with 36 more rows

Explanation of command

We take the full data for one participant (dat) and then filter it to retain only the rows marked ER (the expectancy rating rows, see codebook.md). We then add a new column (add_column) that is called trial and contains the numbers 1 to 46 (1:46). Finally, we select just the subj, trial, and value columns of that dataframe, and place them into expect.

Explanation of output

The dataframe expect contains one row for each trial, and the column value contains the expectancy rating for that trial.

Combining expectancy and GSR data

In the final step of preprocessing this participant, we join the expectancy ratings expect and the mean GSRs (gsr.sum) into a single data frame:

## Combine GSR and expectancy data
comb.data  <- full_join(gsr.sum, expect, by = "trial")
comb.data
# A tibble: 46 x 5
# Groups:   trial [46]
   trial   run    cval  subj value
   <int> <dbl>   <dbl> <dbl> <dbl>
 1     1     0  0.361    168     2
 2     2    -1 -0.109    168     3
 3     3     1  0.181    168     3
 4     4     0  0.125    168     2
 5     5    -2  0.0404   168     4
 6     6     1  0.0714   168     4
 7     7     0 -0.252    168     3
 8     8     0  0.644    168     3
 9     9    -3  0.427    168     4
10    10     0  0.874    168     4
# … with 36 more rows

Explanation of command

The command full_join joins together two separate data frames, using a column they have in common. In this case, the gsr.sum and expect data frames have the trial column in common.

List of participant numbers

We’ve now completed the preprocessing of one participant (168). In principle, we could go through all the same steps for each of the other approximately 50 participants, but that would be rather tedious. Instead, we can use a loop to automate this for us, as we’ll see in a minute, but first we need to get a list of participant numbers

## Create list of participant numbers
subjs <- enframe(unique(raw$subj))
subjs
# A tibble: 43 x 2
    name value
   <int> <dbl>
 1     1   200
 2     2   198
 3     3   197
 4     4   196
 5     5   195
 6     6   193
 7     7   191
 8     8   188
 9     9   186
10    10   184
# … with 33 more rows

Explanation

We need a list of the different participant numbers used in this experiment. We can get this using the subj column of the raw data, so raw$subj. The only problem is that each subject number occurs many times (once for each GSR recording, so 20 times a second). We want just one copy of each different subject number. The command unique does that for us, so the command subjs <- enframe(unique(raw$subj)) gives us the participant numbers (enframe just means ‘put in a data frame’).

More on loops

We now use a loop to go through the same set of steps for each participant. This takes a little while to run, because it preprocesses 2,300 trials (46 trials for each of 50 participants);

## Baseline correct all trials for all participants
sum.data  <- NULL
for(this.subj in subjs$value) {
    dat  <- raw %>% filter(subj == this.subj)
    gsr <- dat %>% filter(key == "GS")
    cs <- dat %>% filter(key == "CO")
    onep  <- NULL
    for(trial in 1:46) {
        onep  <- bind_rows(onep, base.correct(trial))
    }
    gsr.sum  <- onep %>%
        filter(rtime >= 0) %>%
        group_by(trial, run) %>%
        summarise(cval = mean(cval))
    expect <- dat %>%
        filter(key == "ER") %>%
        add_column(trial = 1:46) %>%
        select(subj, trial, value)
    comb.data  <- full_join(gsr.sum, expect, by = "trial")
    sum.data  <- bind_rows(sum.data, comb.data)
}
sum.data
# A tibble: 1,978 x 5
# Groups:   trial [46]
   trial   run    cval  subj value
   <int> <dbl>   <dbl> <dbl> <dbl>
 1     1     0  0.334    200     1
 2     2    -1 -0.118    200     4
 3     3     0  0.173    200     2
 4     4     0  0.124    200     4
 5     5     3  0.125    200     4
 6     6     0 -0.0832   200     2
 7     7     0  0.228    200     4
 8     8    -3  0.228    200     4
 9     9     1  0.0773   200     5
10    10     0  0.789    200     4
# … with 1,968 more rows

Explanation of output

Our full set of preprocessed data, one row per participant per trial. We’ll use this in our analyses in the next step.

Explanation of commands

You can safely ignore everything apart from the first two lines and the last three lines of commands. This is because the bit in the middle is exactly the same commands as we’ve been building up throughout this worksheet. They take one participant, baseline correct all their GSRs, take a mean GSR for each trial, and record that along with the expectancy rating, and information about trial (participant number, trial number, run length).

All of these commands are placed inside a for loop, which we’ve used before in this worksheet. We use the set of participant numbers (subj$value) we just collected and use the for loop to set this.subj to each of these values in turn. All the data is combined together in sum.data, using the bind_rows command we’ve used before.

Log transform

There are three further preprocessing steps we have to take before we can analyze these data. First, studies of GSR data normally log transform it before analysis. Log transforming data means to take the logarithm of the data, in this case the natural logarithm. Two reasonable questions about this preprocessing are: (a) what does that mean, and (b) why would you do it?

To answer the first question, a log transform compresses the range of the data. The following graph should help visualize this:

## Visualizing logarithms
plot(1:50,log(1:50))

On the x-axis we have the numbers 1 to 50. On the y-axis, we have the natural logarithm of those numbers. As you can see, while the x-axis rises from 1 to 50, the y-axis rises from zero to around 4. You should also be able to notice that for each increase in x, the increase in y gets smaller. So, for example, where x rises from 1 to 2, log(x) rises from 0 to about .8; but where x rises from 2 to 3, log(x) rises by about 0.4.

As a result, a log transform doesn’t change small numbers very much, but reduces large numbers a lot more. This can be useful if, for example, a few participants have unusually large GSRs. These very large GSRs would increase the variance of the data substantially, and this can make it harder to detect real differences between conditions. A log transform can reduce that problem, and hence can make it easier to find real effects in such situations. So, this also answers the second part of the question - we apply a log transform to GSR data because it can make it easier to find real effects.

We log transform our GSR data using the mutate command that we used earlier:

## Log transformation of GSR
sum.data  <- sum.data %>% mutate(lcval = log(cval+1))

The command log takes the natural logarithm. We calculate log(cval+1) rather than log(cval) because the log(0) is an infinitely small number, which is something that cannot be analyzed. By adding 1 we don’t hit this problem (unless cval is -1 or smaller, which it seldom is).

Removing trials

In our Perruchet Effect experiment, we have runs of various lengths. For example, for something to be a +2 run there must be a sequence of four trials in the following order NSSN where N means no sound, and S means sound. The third trial in that sequence is a +2 trial, because there have been two sounds in a row, and on the following trial there will not be a sound so the run will end. Importantly, at least the way people normally analyse these data, the second trial in that sequence has no definable run length. One might say it is a +1 trial because, at that point, there has only been one sound in a row. However, the experimenter knows that the run will continue on the next trial, so calling it a +1 trial somehow doesn’t seem quite right (although, from the perspective of the participant, at the time they are reacting to that trial, clearly it is a +1 trial).

Normally, these experimenter-ambiguous trials are described as having a zero run length, and are excluded from the analysis. Run length is recorded directly into the data file itself, so we can remove zero run-length trials like this:

## Remove trials where run length is zero
sum.data  <- sum.data %>% filter(run != 0)

This is a standard filter command. The != part reads as ‘not equal to’. So, this command keeps all the non-zero run lengths.

Recoding data

Another convention of analyzing Perruchet effect experiments is that you convert run length (-3 to +3) into a dependent variable called “Level”. This involves coding -3 and +1 as Level 1, -2 and +2 as Level 2, and -1 and +3 as Level 3. If GSR rises from -3 to +3 run lengths, it will also rise from Level 1 to Level 3. Equally, if GSR drops from -3 to +3, it will also drop from Level 1 to Level 3. The reasons for recoding the dependent variable in this way are not straight forward, and are a matter of some debate. Nonetheless, as this experiment is essentially an attempt to replicate McAndrew et al.’s paper with an unpleasant noise, we chose to use the same analyses they did, and that involves recoding the run length as Level.

We covered recoding data in the Cleaning up questionnaire data part of Research Methods in Practice (Quantitative Section). To recode some data, we need to take two steps. First, we define the mapping - so, for each run length, what Level is that? We do this as follows:

## Define mapping of run length to Levels
level.code  <- c(`-3` = 1, `-2` = 2, `-1` = 3, `1` = 1, `2` = 2, `3` = 3)

So, for example, a run length of -2 becomes a Level of 2.

Once we have our mapping, we can use it, via mutate to create a new column called level. We use the recode command to do this, converting the column run using the mapping level.code we’ve just written. The !!! part probably isn’t worth spending the time to understand deeply, but if you’d like to dig deeper, see the explanation in Research Methods in Practice (Quantitative Section).

## Recode run length as Levels
sum.data <- sum.data %>% mutate(level = recode(run, !!!level.code))

Finally, we create a new participant-level summary of our data, using level (rather than run, as sum.data did):

## Summarize GSR and expectancy, by Level, for each participant
lvl.sum  <- sum.data %>%
    group_by(subj, level) %>%
    summarise(lcval = mean(lcval), expect = mean(value))

Reporting means

The preprocessing is done! As a first, quick test of our hypotheses, we can just take a peek at the mean GSR and expectancy rating, across all participants, for each Level. We use the familiar group_by and summarise commands to do this:

## Overall mean GSR and expectancy, by Level
mdata  <- lvl.sum %>% group_by(level) %&