Study Design: I showed participants some information about sea-level rise, focusing the information in different ways, both in terms of the time-scale and the magnitude of potential rise. Thus I had a 2 (Time: 2050 or 2100) by 2 (Magnitude: Medium or High) design. There were also two control groups who received no information, only answering the questions for my DVs.
Questions: I've always checked for normality within cells -- for the 2x2 portion of this design, it would mean looking for normality within 4 groups. However, reading some discussions here has made me second guess my methods.
First, I've read that I should be looking at the normality of the residuals. How can I check for normality of residuals (in SPSS or elsewhere)? Do I have to do this for each of the 4 groups (6 including the controls)?
I also read that normality within groups implies normality of the residuals. Is this true? (Literature references?) Again, does this mean looking at each of the 4 cells separately?
In short, what steps would you take to determine whether your (2x2) data are not violating assumptions of normality?
References are always appreciated, even if just to point me in the right direction.