But taking up Art's point, why not use a two-factor ANOVA model, with Gender as a between-subjects factor, and time (pre & post) as a within-subjects (or repeated measures) factor? That model will let you test the main effects of Gender and Time as well as their interaction. The null hypothesis for the interaction term is that the pre-post difference (which seems to be your main interest) is the same in both groups. If you get a statistically significant F-test for the interaction, you can reject that null hypothesis.
The most common way to run that model is via Analyze - GLM - Repeated Measures. You have to enter some info for the repeated measures variable in the first dialog that comes up. Name the within-Ss factor Time, and indicate that there are 2 levels, then click ADD followed by Define.
In the next dialog that appears, move your pre and post variables into the appropriate spots in the top box, and add gender to the Between-S factors box. (It probably has to be a numeric variable, so if you have it as a string, you'll have to recode it first.)
A plot is useful in helping you understand the nature of the interaction (if there is one). You want Time as the horizontal axis with separate lines for Gender.
You probably want the "estimated marginal means" too. Click on Options to bring up the dialog that allows you to select those. The most common approach is to select all of them except the Overall, which is usually of less interest than the others.
Exit the main dialog via the PASTE button, and you should see some syntax that looks something like this:
GLM pre post BY gender
/WSFACTOR=Time 2 Polynomial
/METHOD=SSTYPE(3)
/PLOT=PROFILE(Time*gender)
/EMMEANS=TABLES(gender)
/EMMEANS=TABLES(Time)
/EMMEANS=TABLES(gender*Time)
/CRITERIA=ALPHA(.05)
/WSDESIGN=Time
/DESIGN=gender.
Highlight it and run it (via one of the Run options on the toolbar, or by CTL-R).
In the output, the main effect of Time and the Time*Gender interaction will appear in the Tests of Within-Ss Effects box, and the main effect of Gender will appear in the Tests of Between-Ss Effects box. (I might not have those labels exactly right, but they're close.)
Finally, you should be able to find lots of online tutorials on how to run this model. Try searching on <SPSS mixed design ANOVA> or <SPSS split-plot ANOVA> or <SPSS between-within ANOVA>, for example.
HTH.
StatsUser wrote
Thank you Rich (and all who took time to respond),
That was the problem! I no longer get the warning message, many thanks for your help.