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Self-selection into online or face-to-face studies

December 13, 2010

A new paper by Edward Witt, Brent Donellan, and Matthew Orlando looks at self-selection biases in subject pools:

Just over 500 Michigan State University undergrads (75 per cent were female) had the option, at a time of their choosing during the Spring 2010 semester, to volunteer either for an on-line personality study, or a face-to-face version…

Just 30 per cent of the sample opted for the face-to-face version. Predictably enough, these folk tended to score more highly on extraversion. The effect size was small (d=-.26) but statistically significant. Regards more specific personality traits, the students who chose the face-to-face version were also more altruistic and less cautious.

What about choice of semester week? As you might expect, it was the more conscientious students who opted for dates earlier in the semester (r=.-.20). What’s more, men were far more likely to volunteer later in the semester, even after controlling for average personality difference between the sexes. For example, 18 per cent of week one participants were male compared with 52 per cent in the final, 13th week.

Self-selection in subject pools is not a new topic — I’ve heard plenty of people talk about an early-participant conscientiousness effect (though I don’t know if that’s been documented or if it’s just lab-lore). But the analyses of personality differences in who takes online versus in-person studies are new, as far as I know — and they definitely add a new wrinkle.

My lab’s experience has been that we get a lot more students responding to postings for online studies than face-to-face, but it seems like we sometimes get better data from the face-to-face studies. Personality measures don’t seem to be much different in quality (in terms of reliabilities, factor structures, etc.), but with experiments where we need subjects’ focused attention for some task, the data are a lot less noisy when they come from the lab. That could be part of the selection effect (altruistic students might be “better” subjects to help the researchers), though I bet a lot of it has to do with old-fashioned experimental control of the testing environment.

What could be done? When I was an undergrad taking intro to psych, each student was given a list of studies to participate in. All you knew was the codenames of the studies and some contact information, and it was your responsibility to arrange with the experimenter to take the experiment. It was a pain on all sides, but it was a good way to avoid these kinds of self-selection biases.

Of course, some people would argue that the use of undergraduate subject pools itself is a bigger problem. But given that they aren’t going away, this is definitely something to pay attention to.

  1. December 13, 2010 5:45 pm

    Interesting post!
    I’ve heard colleagues comment that neuroticism, anxiety, and depression scores are higher later in semester when doing research with research participation pool students.
    Is it the pressure of assessment?
    Is it differences in the types of students who get in earlier or later?
    I suppose it wouldn’t be hard to tease the effects out.

  2. Sanjay Srivastava permalink
    December 14, 2010 11:09 am

    These researchers didn’t find that week correlated very much with neuroticism (r=-.08, and unless I’m misreading how they coded time, the effect in the opposite direction of what you’re describing, but it wasn’t significant). But it would be interesting to see what happens in different classes and at different institutions. If it’s a pressure thing, the effects you’re describing might be more pronounced in a class that has an all-or-nothing comprehensive final exam (or at an institution where such exams are the norm for all classes).

  3. Tobi Ceh permalink
    April 7, 2011 7:58 pm

    Do you have a reference for this paper?

  4. Sanjay Srivastava permalink
    April 8, 2011 8:22 am

    It’s at the bottom of the BPS Digest link. (I know, I know — it’s weird and disorienting for a popular writeup to actually have the reference!)

    Witt, E., Donnellan, M., and Orlando, M. (2011). Timing and selection effects within a psychology subject pool: Personality and sex matter. Personality and Individual Differences, 50 (3), 355-359

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