I’m exploring if hair cortisol levels (pg/mg) are related with executive functions like working memory (measured on WISC-scale), inhibition control (BRIEF-scale) and attention shifting (BRIEF-scale) with SPSS. My sample sizes are 11 (group 1) and 21 (group 2). I want to study the groups together (are cortisol and executive functions related) and separately (is the relationship different in different groups). The hair cortisol values are positively skewed.
Other studies in this field have used log transformation of the (hair) cortisol without bootstrapping, but I want to use bootstrapping because of the small sample size and I’m now wondering whether to log transform the data before bootstrapping or not?
Is the regression analysis right method for estimating the relationship between cortisol and executive functions? If not, what alternatives could you recommend? My hypothesis is that the relationship is U-shaped (very low and very high cortisol levels are related with more problems in executive functions), so I would add cortisol to the model in the first step, and squared cortisol in the model to the second step.
I want to study if the sex, age and grouping variable have an impact on the relationship of working memory and cortisol. Can I explore this with a general linear model?
I would really appreciate for any feedback!