I am using SPSS and having some trouble with a research question which is analogous to the hypothetical question:
Is there a longitudinal Relationship between Happiness and Chocolate Consumption?
Let’s say I take a sample of people and contact them when they are aged 14 and aged 18 and ask them: a) What is your chocolate consumption in grams per day. b) Are you happy?
I have my fictitious data in the following wide format:
ID HAPPY.14 HAPPY.18 CHOC.14 CHOC.18
1 YES YES 100 5
2 YES NO 50 30
3 NO YES 30 50 etc.
I would like to know if the mean chocolate consumption per day is higher among happy people than those who are not happy while accounting for the fact that I have taken repeated measures of both chocolate and happiness at the two time points.
I suppose one way of doing this would be to do an ANCOVA, using time (before/after) as a grouping variable and controlling for happiness status. However, I think this may be inadvisable as correlation between the two time points would be neglected.
I understand that one valid approach for this should be a repeated measures ANOVA. I’m just not sure how to do this correctly in SPSS. I have specified my within-subjects factor as chocolate consumption with age 14 data as level one and age 18 data as level 2. What I’m uncertain about is the next step – specifying covariates and between individual factors – I have the option of adding HAPPY.14 and OR HAPPY.18 as a between individual factor. If I add both the output tells me about the effect of HAPPY.14 and HAPPY.18 as you’d expect, not about the “effect” of happy (YES/NO)per se.
I realise it’s a basic question. Any feedback would on either of the two approached would be greatly appreciated.