Empirically Based Characteristics of Effect Sizes used in ANOVA J. Jackson Barnette, PhD Community and Behavioral Health College of Public Health University of Iowa

Examine characteristics of four commonly used effect sizes Standardized Effect Size Measures of Association: –Eta-Squared –Omega-Squared –Intraclass Correlation Coefficient

Standardized Effect Size It represents mean differences in units of common population standard deviation. Population FormStatistic Form  1 – µ 2 X 1 – X 2  = d=  s In practice, the Std. Deviation is typically replaced with the Root Mean Square Error

Standardized Effect Size in ANOVA Mean Range Std. Effect Size = MSE

Cohen’s Standards Cohen needed to base his research on power on some effect sizes so he pretty much arbitrarily chose three values that had been used extensively as standards for effect sizes:.2 is a “small effect”.5 is a “moderate effect”.8 is a “large effect”

Observed Effect Sizes when K= 2 n= 5, mean=.55, sd=.47, p>.2=.76, p>.5=.44, p>.8=.24 n= 30, mean=.21, sd=.16, p>.2=.44, p>.5=.06, p>.8=.00 n= 60, mean=.15, sd=.11, p>.2=.27, p>.5=.01, p>.8=.00 n=100, mean=.11, sd=.09, p>.2=.16, p>.5=.00, p>.8=.00

Observed Effect Sizes when K= 4 n= 5, mean=.97, sd=.46, p>.2=.99, p>.5=.85, p>.8=.59 n= 30, mean=.38, sd=.16, p>.2=.87, p>.5=.22, p>.8=.01 n= 60, mean=.29, sd=.11, p>.2=.70, p>.5=.03, p>.8=.00 n=100, mean=.21, sd=.09, p>.2=.49, p>.5=.00, p>.8=.00

Observed Effect Sizes when K= 10 n= 5, mean= 1.40, sd=.40, p>.2= 1.00, p>.5= 1.00, p>.8=.96 n= 30, mean=.56, sd=.15, p>.2= 1.00, p>.5=.64, p>.8=.06 n= 60, mean=.40, sd=.10, p>.2=.99, p>.5=.15, p>.8=.00 n=100, mean=.31, sd=.08, p>.2=.92, p>.5=.00, p>.8=.00

Eta-Squared (Pearson and Fisher) SS treatment  2 = Ss total A  2 of.25 would indicate that 25% of the total variation is accounted for by the treatment variation.

Eta-Squared Positives: easy to compute and easy to interpret. Negatives: it is more of a descriptive than inferential statistic, it has a tendency to be positively biased and chance values are a function of number and size of samples.

The Bias in Eta-Squared Mean sampled  2 Sample Size K

Omega-Squared (Hays, 1963) When a fixed effect model of ANOVA is used, Hays proposed more of an inferential strength of association measure, referred to as Omega-Squared (  2 ) to specifically reduce the recognized bias in  2. It provides an estimate of the proportion of variance that may be attributed to the treatment in a fixed design.  2 =.32 means 32% of variance attributed to the treatment.

Omega-Squared  2 is computed using terms from the ANOVA SS treatment – ( K – 1) MS error  2 = SS total – MS error

Omega-Squared Positives and Negatives (Pun intended) of  2 Positives: it is an inferential statistic that can be used for predicting population values, easily computed, it does remove much of the bias found in  2. Negatives: it can have negative values, not just rounding error type, but relatively different than 0. If you get one that is negative, call it zero.

Intraclass Correlation Omega-squared is used when the independent variable is fixed. Occasionally, the independent variable may be “random” in which case the intraclass correlation is used to assess strength of association.

Intraclass Correlation Values to determine the ICC come from the ANOVA. MS treatment – MS error  I = MS treatment + ( n – 1) Ms error The ICC is a variance-accounted-for statistic, interpreted in the same way as is Omega-Squared. It also has the same strengths and weaknesses.