Jeromy Anglim's Blog: Psychology and Statistics

Friday, February 27, 2009

Books on Writing, Grammar, and Style in Psychology and Beyond

This post lists some books on academic writing, which I've found useful.

Thursday, February 19, 2009

Assorted issues with Multiple Regression and Moderation

1. Do variables in moderation have to be normally distributed before centering? 
No, normality is not a requirement. However, you could apply a transformation if you felt it was justified, such as when it better reflects the meaning of the variable.
See the following for a discussion of issues in transformations: 

Assumptions in moderation are the same as in normal multiple regression. They mainly concern the pattern of the residuals. For a discussion see:

2. Do I use the R squared or adjusted R squared when reporting a multiple regression?
You can use either or both. The larger your sample, the smaller the difference is between adjusted R square and R-squared. R-squared is a biased estimate.

3. p=0.52, is that still significant?
Jacob Cohen once said something along the lines that surely God must love .06 almost as much as .04. 
You can write p = .05
Whether this is statistically significant depends on your alpha. If your alpha is .05, then your obtained p value is not less than .05, and thus, it is not statistically significant. Reflecting the seemingly arbitrary cut-off consequences off null hypothesis significance testing, researchers often write that there was a trend toward significance when p <.10 and >=.05.

4. Would a significance level of 0.06 be slightly significant or not at all?
Trend toward significance is a common label, see above.

5. Do you have any examples of how to write up a moderator regression? 
Frazier, P., Tix, A., & Barron, K. (2004). Testing moderator and mediator effects in counseling psychology research. Journal of Counseling Psychology, 51, 115-134.

McAlister, A. R., Pachana, N., & Jackson, C. J. (2005). Predictors of young dating adults' inclination to engage in extradyadic sexual activities: A multi-perspective study. BRITISH JOURNAL OF PSYCHOLOGY, 96(3), 331.
This article is online at:

Single Group Correlational Study: Basic Analyses

This post sets a basic procedure for analysing a single group observational study in psychology. It aims to provide a starting point, particularly for researchers who do analyse data infrequently.

Formatting Correlation Matrices in Psychology

Researchers in psychology often want to present a correlation matrix of the main variables in a study. This post sets out one way of producing a formatted correlation matrix that conforms to APA style.