## Calculating Within-group variance

My study is looking at attitudes towards a concept across four different professional groups: Physicians, Nursing, Pharmacy, and Allied Health. I want to see whether there are differences in attitudes between the groups (e.g. across the professions) as well as within the groups (amongst members of the same profession). I used a validated survey instrument, comprised of 27 likert-type items, from which I extracted three components using PCA. I created each component as a new variable in SPSS, by averaging the mean of the items that comprised the component (e.g. Variable 1= average of means of questions 1 to 11; variable 2 = average of means of questions 12 to 24, etc.). To get at the between-group results, I’ve done ANOVA/Welch’s ANOVA and the relevant post-hoc tests (Tukey’s HSD, Games Howell, etc.) to determine where there are statistically significant differences in the mean between the four groups, for each of the three new variables. For example, between physicians-nursing, or between pharmacy-nursing.

I now also want to determine if there are differences in attitudes WITHIN each group. What I mean by this is, amongst all of the, for example, physicians, is there significant difference in the means of all physician respondents, for a particular component? So, I am not comparing two or more groups, but rather, want to look at the variance within a single profession group. Can I even do that? I through that interpreting the SD might give me this, but how do you determine if an SD value is statistically significant?