Re: analyzing change for one dichotomous variable, assessed at pre and post

Posted by Bruce Weaver on May 05, 2012; 5:37pm
URL: http://spssx-discussion.165.s1.nabble.com/analyzing-change-for-one-dichotomous-variable-assessed-at-pre-and-post-tp5688136p5688233.html

"goal is to determine if the percentage of participants who changed from Y smoking at pre to N smoking at post is significant."

Significantly different from what?  

The usual analysis for the situation you describe would be the McNemar chi-square test, aka the McNemar Change Test.  The data are in a 2x2 table with Yes & No for Pre in the rows, and Yes and No for post in the columns.  McNemar's chi-square is equivalent to a chi-square goodness of fit test on the two discordant cells,* with a null hypothesis specifying the same number of changes of each type (Y to N and N to Y).  (If the cell counts are too low, you can use a binomial test instead--SPSS computes one in that case, IIRC.)  Does that address the question you have in mind?  

* McNemar's test does not use the data in the concordant cells on the main diagonal.

pji wrote

I have a situation where I have one variable, smoking yes/no, taken at pre then taken at post, few years later. n ~70. same participants at both pre and post. the possible combinations are

smoking     pre       post
                 y          y
                 n          n
                 y          n
                 n          y

goal is to determine if the percentage of participants who changed from Y smoking at pre to N smoking at post is significant.

I don't think any sophisticated analysis has to be conducted. I would simply assign the four possible outcomes to four categories (y - y, n - n, y - n, n - y) and then run a chi-square to determine if the observed frequency counts of those four categories are different from the expected frequency counts. there is no post-hoc test, so I would have to heuristically examine which categories have the most deviance.

are there any analyses that could be conducted?
thank you in advance.
pj