Effect Size – Ch. 4 Effect Size is to Difference Between Means as Z i is to X i An easier way to assess the magnitude of X-bar 1 -Xbar 2 Looks at the difference between means relative to differences between individuals within each group Are groups differences large relative to individual differences? E.G.: Cognitive ability is 13 points higher in first-born relative to second-born children Is this an important difference?

Effect Size, cont. Numerator: Difference Between Groups (Means) Denominator: Differences Among Subjects Within each Group Ratio: Is a Comparison of Differences Between to Differences Within (Absolute Value) Think Ahead: t-tests and ANOVAs compare differences Between To differences Within

Large Effect d is large  --13 pt. Difference-- 

Small Effect d is small 13 pt. Difference  

The Two Ts of Research: True but Trivial TT

Interpreting d Small Effect:d=0.20 Medium Effect:d=0.50 Large Effect:d=0.80

What Does p<0.05 Mean? “Significant”: Signifies a relationship, does not imply that that relationship is large or important.