You can use a conventional paired t test. Although you have dichotomous
scores that does not mean they are categorical. Correct/incorrect is a
ratio scale of 1 unit.
Green/Red, Accountant/Psychologist are the type of categorical
dichotomies that bring in the nonparametric procedures like Chi-square
or ranking tests.
Just calculate a mean difference and variance for each item and analyze
them the usual way. You might also try some of the test reliability
stats that are now in
SPSS, such as coefficient alpha. Alpha is a general index of how well
the items intercorrelate or "hang together".
Mike Williams
----- Original Message -----
From: "Annette Taylor"<[email protected]>
To: "Teaching in the Psychological Sciences (TIPS)"<[email protected]>
Sent: Tuesday, January 15, 2013 6:21:42 PM
Subject: [tips] my crummy knowledge of stats
I know this is a basic question but here goes:
I have categorical data, 0,1 which stands for incorrect (0) or correct (1) on a
test item.
I have 25 items and I have a pretest and a posttest and I want to know on which
items students improved significantly, and not just by chance. Just eyeballing
the data I can tell that there are some on which the improved quite a bit, some
not at all and some are someplace in the middle and I can't make a guess at
all. That is why we have statistics. Yeah! .... hmmmm....bleh.....
As far as I know, the best thing to do is a chi-square test for each of 25
items; but of course that will mean that with a .05 sig level I will have at
least one false positive, maybe more, but most assuredly at least one. This
seems to be a risk. At any rate I can use SPSS and the crosstabs command allow
for calculation of the chi-square.
I know that when I do planned comparisons with multiple t-tests, I can do a
Simes' correction in which I can rank order my final, obtained alphas, and
adjust for the number of comparisons and reject from the point from which the
obtained alpha failed to exceed the corrected-for-number-of-comps alpha. But as
far as I know, I cannot do that with 25 chi square tests. There is probably
some reason why I can no more do that, that relates to the reason for why I
cannot do 25 t-tests in this situation with categorical data.
Is there a better way to answer my research question? I need a major professor!
Oh wait, that's me... drat! I need to hire a statistician. Oh wait, I'd need $$
for that and I don't have any. So I hope tipsters can stand in as a
quasi-hired-statistician and help me out.
Oh, I get the digest. I don't mind waiting until tomorrow or the next day for a
response, but a backchannel is [email protected]
I will be at APS this year. Any other tipsters planning to be there? Let's have
a party! I'd love to put personalities to names.
Thanks
Annette
Annette Kujawski Taylor, Ph. D.
Professor, Psychological Sciences
University of San Diego
5998 Alcala Park
San Diego, CA 92110
[email protected]
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