I am trying to analyze whether different intervention groups scored differently in questionnaire measures before and after an intervention.
The first independent variable is time at 2 levels (pre-intervention and post-intervention). The second independent variable is Intervention group at 3 levels.
I also want to include age and gender as covariates in order to control for them. Gender and age have both been included as a numeric variable type within SPSS as when i tried to code gender as a string variable type (e.g. M=1, F=2) it would not let me analyse the data.
I try to use a general linear model --> Repeated measures and then classify my within subjects factor as the scores on the two questionnaires at 2 levels. The intervention is put in as the Between-subjects factor and the gender and age are included as covariates.
The problems I am having:
- Have I encoded gender into SPSS correctly for this kind of analysis?
- Am I doing the correct kind of analysis for what I am trying to achieve?
- Without including the covariates, there is a significant difference between pre-intervention and post-intervention (no significant interaction with the groups) however when including the covariates, intervention * gender is now only significant but the intervention is no longer significant. What does this mean?
Any help would be appreciated.