Effect Size: What is it and how might we use it?

What is Effect Size and how is it measured?
What Is Effect Size?
An effect size can be defined as the degree to which a phenomenon is present in a population – thus, the magnitude of an intervention’s effect or impact (Cohen, 1988; Kline, 2004). There are two major types of effect sizes:
  • One is appropriate for research that is experiment in nature such as those based on comparing groups. For example, a class that received a treatment, such as reciprocal teaching, and another class that did not receive the intervention.
  • The other effect size approach is used to establish the impact of an intervention over item (pre-post).
They have different interpretations and it is an empirical question as to whether they differ.  In Visible Learning (2009) the majority of the effects sizes were based on research that investigated group comparisons.

Why Use Effect Size Instead of Other Statistics?
The statistic places emphasis on impact of the difference found amongst samples, without being confounded by the size of the samples being compared. Although tests of statistical significance also estimate the size of the effect, the result is affected by the size of the sample. An effect size is a standardized and scale-free measure of the relative size of the effect of an intervention.

How Do You Calculate Effect Size?
There are various ways that you can calculate an effect size, and the choice really depends on what effect is being assessed – either assessing the same students pre- and post-intervention effect over time, or when the comparison relates to comparing those that received the intervention (experimental group) against those that did not (control group).

When using the Progress Achievement Tool, the effect size is calculated by taking the difference between the averages of the post-assessment and pre-assessment, then dividing by the average of their standard deviations.

​Hattie Ranking: 252 Influences And Effect Sizes Related To Student Achievement

How might we use effect size to measure the effectiveness of our teaching strategies?
Backup of Hattie’s Ranking list of 256 Influences And Effect Sizes Related To Student Achievement (PDF)
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