Appendix: Interpreting Effect Sizes and Reporting p-Values

17.6 Interpretation of \(\eta^2\) and \(r^2\) (Cohen, 1988)

These are reference points, not firm cutoffs. For example, .056 is a medium effect size.

Effect Size	Interpretation
\(\eta^2 = r^2 = .01\)	Small effect
\(\eta^2 = r^2 = .06\)	Medium effect
\(\eta^2 = r^2 = .14\)	Large effect

17.7 Interpretation of \(r\) (Cohen, 1988; Note: This is not \(r^2\))

These are reference points, not firm cutoffs. For example, .056 is a medium effect size.

Effect Size	Interpretation
\(r\pm.10\) Small effe	ct
\(r\pm.30\) Medium e	ffect
\(r\pm.50\) Large effe	ct

17.8 Interpretation of d (Cohen, 1988)

These are reference points, not firm cutoffs. For example, .45 is a medium effect size.

Effect Size	Interpretation
\(d = \pm.2\)	Small effect
\(d = \pm.5\)	Medium effect
\(d = \pm.8\)	Large effect

17.9 Interpretation of \(\phi\) (phi; Cohen, 1988)

These are reference points, not firm cutoffs. For example, .29 is a medium effect size.

Value of \(\phi\)	Effect size
\(\phi\pm.10\)	Small effect
\(\phi\pm.30\)	Medium effect
\(\phi\pm.50\)	Large effect

17.10 Reporting p-Values from SPSS

In your results paragraphs, you will need to report p-values. SPSS labels p-values “Sig.” In APA-style, report the exact p-value. Examples:

SPSS Reports	Your results paragraph
Sig. = .032	p = .032
Sig. = .051	p = .051
Sig. = .731	p = .731

There is one exception. If the \(p\)-value drops below .001, SPSS will cut off the trailing digits and show the value as “.000.” This is not zero! It is a number somewhere less than .001 (really, less than .0005 if you want to get technical). Report those as p < .001:

SPSS Reports	Your results paragraph
Sig. = .000	p < .001

The above example is the only time you should write p-values with anything except an equals sign. The “p < .05” notation is a throwback to the days before we had software to calculate exact p-values. A lot of information is contained in your statistical results, so it’s important to report them accurately.