It seems every editor and her brother these days wants to see standardized effect size statistics reported in journal articles.
For ANOVAs, two of the most popular are Eta-squared and partial Eta-squared. In one way ANOVAs, they come out the same, but in more complicated models, their values, and their meanings differ.
SPSS only reports partial Eta-squared, and in earlier versions of the software it was (unfortunately) labeled Eta-squared. More recent versions have fixed the label, but still don’t offer Eta-squared as an option.
Luckily Eta-squared is very simple to calculate yourself based on the sums of squares in your ANOVA table. I’ve written another blog post with all the formulas. You can
check it out here.
But if you’re still wondering about the details of the differences between partial Eta-squared and Eta-squared and which one you ought to be using, I recommend reading this article:
Levine, T.R. & Hullett, C.R. (2002). Eta Squared, Partial Eta Squared and the Misreporting of Effect Size in Communication Research. Human Communication Research, 28, 612-625.
The article link at the bottom is unfortunately a dead link. However, the article is freely available elsewhere on the web if you search for it by title. For example, I found it on ResearchGate.