- ‘Effect Size’ is used to gauge the difference between the means of two sets of variables. Standard Deviation will be used to determine the magnitude of the difference indicating the strength of the effect. It can be represented by the following formula:
Effect Size = | MeanS – MeanR | / SD
MeanS is the mean of the school data; MeanR is the mean of the norm or the reference data; SD is the Standard Deviation of the norm or the reference data. When ‘Effect Size’ is 0, there is no difference between the two means. The greater the value of ‘Effect Size’ it becomes, the greater and the more significant the difference it makes. According to the value of ‘Effect Size’, the strength of the effect can be classified in the following table:
Effect Size | Description |
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< 0.20 | Negligible |
0.20 – 0.49 | Small |
0.50 – 0.79 | Moderate |
0.80 – 0.99 | Large |
>= 1.00 | Very large |