2022 Introduction to Statistics in Research Mitchell 2nd ed

I N T R O T O R E S E A R C H : D A T A V I S U A L I Z A T I O N & C O M M O N S T A T T E S T S

EFFECT SIZE

SPSS (version 27) provides several options for reporting effect size. Remember, effect size is a measure of practical significance or the importance of the results.

Table 137: Mitchell, J. (2021) Effect Size Advantages [Infographic].

Let’s look at the advantages as reported by Tomczak & Tomczak (2014), and illustrated in the infographic above.

1) They reflect the strength of the relationship between variables. 2) Effect size estimates allows comparisons between difference sources and researchers. 3) They can be used to determine the sample size needed for a study. 4) Effect size from pilot studies may be an indicator of future research results.

SPSS (version 27) will produce several effect sizes. Using the dataset as an example check the box for “Estimate effect size for overall tests” and SPSS will produce eta -squared, epsilon-squared, omega-squared Fixed -effect, and Omega-Squared Random-effect. Eta-squared is used specifically in ANOVA models. “One of the disadvantages of 2 is that the value of each particular effect is dependent to some extent on the size and number of other effects in the design” (Tomczak & Tomczak, 2014, p. 22). Another disadvantage to

eta- squared is that it is a “biased measure of population variance explained, although it is accurate for the sample” (Martin, 2021). A solution is to calculate partial eta-squared. Partial eta-squared is also available in SPSS, but takes a little more effort. A partial eta-squared effect size will normally be lower than eta-squared because it includes the SS Error .

Table 138: Estimate effect size

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