Stroop Test

Quick note: Local server necessary for running d3.js. Computations were done in R.

Background Information

In a Stroop task, participants are presented with a list of words, with each word displayed in a color of ink. The participant’s task is to say out loud the color of the ink in which the word is printed.

The task has two conditions: a congruent words condition, and an incongruent words condition. In the congruent words condition, the words being displayed are color words whose names match the colors in which they are printed: for example style RED, BLUE. In the incongruent words condition, the words displayed are color words whose names do not match the colors in which they are printed: for example PURPLE, ORANGE. In each case, we measure the time it takes to name the ink colors in equally-sized lists. Each participant will go through and record a time from each condition.

Project Questions & Answers

  1. What is our independent variable? What is our dependent variable?
    • Independent (explanatory) variable: condition (i.e., congruence) of a word
    • Dependent (response) variable: time to read a word
  2. What is an appropriate set of hypotheses for this task? What kind of statistical test do you expect to perform? Justify your choices.
    The one-tailed test seems a better option than a two-tailed test if we intuitively consider incongruent words will not improve recognition times. The one-tailed test provides more power to detect an effect although we miss the opportunity to test if incongruent might improve recognition time. It this case, the risks are negligible.

    The hypothesis are thus:
    H0: μCon = μIn
    • Null hypothesis (H0) states that there is no difference between incongruent and congruent population mean times
    • μCon denotes the congruent population mean and μIn denotes the incongruent population mean
    HA: μCon < μIn
    • Alternative hypothesis (HA) states that the congruent population mean time is smaller than the incongruent mean time

    We are going to choose a dependent t-test:

    • T-test because we don't know any population parameters (e.g. standard deviation) and the sample is fairly small (n < 30). Besides, our distributions are nearly bell-shaped distributions. Such distributions "will still provide values of T that approximate the t-distribution very closely."(Walpole, R. et al. 2011, p. 248)
    • Dependent because this is similar to a "before and after test" – a repeated mesaurement with each participant. We test the mean of pairwise differences.
  3. Report some descriptive statistics regarding this dataset. Include at least one measure of central tendency and at least one measure of variability.
    vars n mean sd median min max range se
    Congruent 24 14.05 3.56 14.36 8.63 22.33 13.70 0.73
    Incongruent 24 22.02 4.80 21.02 15.69 35.26 19.57 0.98
  4. Provide one or two visualizations that show the distribution of the sample data. Write one or two sentences noting what you observe about the plot or plots.

    • The incongruent distribution has two outliers in the far right at roughly 35 seconds
    • Other than that, both distributions resemble a normal distribution

  5. Now, perform the statistical test and report your results. What is your confidence level and your critical statistic value? Do you reject the null hypothesis or fail to reject it? Come to a conclusion in terms of the experiment task. Did the results match up with your expectations?
    • Confidence level = 0.95 %
    • P-critical = 0.5 %
    • T-critical = -1.71
    • Our result is: t = -8.0207, df = 23, p-value = 2.052e-08
    • With a p value close to zero we can confidently reject the null hypothesis. In other words, we can say that it takes significantly longer to complete the incongruent condition task than the congruent condition task.
    • The result matches our expectations (i.e., our intuition and the above visualization of the distributions).
  6. Optional: What do you think is responsible for the effects observed? Can you think of an alternative or similar task that would result in a similar effect? Some research about the problem will be helpful for thinking about these two questions!

    When the words are incongruent the brain has to perform two tasks. First, it sees the colour of the word and subconsciously also thinks this matches the written word. Second, in a separate task, our brain has to focus on the written and try to ignore the colour of the word. In other words, incongruent words just need more “brain power” and hence it takes longer to read the word correctly.
  7. References
    • Feedback from Udacity-Reviewers
    • Walpole, R., Myers, R., Myers, S. Ye, K. 2011. Probability and Statistics for Engineers and Scientists (9th Ed). London: Pearson.