Understanding Effect Size In John Hattie’s Research

December 7th, 2015 § 9 comments

Part 1

As I begin my research and study John Hattie’s claim that Direct Instruction has a far greater effect size compared to Inquiry-Based Learning or Project-Based Learning, I thought it was best to first make sense of what is meant by “effect-size.”

When understanding effect size according to John Hattie it is best to think of a sliding scale from -1 to 1 (although values over/under this are possible). An effect size of 0 would mean that a particular method has no effect on achievement, a negative score means it actually reduces achievement, and a positive score means it increases achievement. To get specific, an effect size of 1.0 would mean that a child’s achievement would advance 2-3 years or increase their learning rate by 50%.

This seems hard to believe as an advancement of 2-3 years seems insane considering Hattie does list several methods that have a 1.0 effect size or higher – like self reporting grades. “Self-report grades” is actually listed as an effect size of 1.44. Does this mean that employing this method would almost double your student’s achievement or raise the student achievement level by more than 3 years?

It quickly becomes clear that understanding Hattie’s effect size calculations seem to have too wide of an interpretation. He goes on to say that an effect size of 1.0 can be interpreted 4 different ways:

  1. A child’s achievement would advance 2-3 years.
  2. Their learning rate would increase by 50%.
  3. A correlation between a variable (like the amount of homework) and achievement was approximately a rate of .5 or 50%.
  4. Students would exceed 84% of students not receiving the “treatment” or method.

These interpretations do not appear to be the same yet could all possibly be used to interpret a 1.0 effect size. He attempts to make it more understandable to a layman by paraphrasing a statistician who helped to originally craft the idea of effect size for the social sciences: Jacob Cohen. Cohen describes the effect size of 1.0 to be like the height difference between a person 5’3″ and someone else who is 6’0″. He is obviously illustrating that the difference is drastic and easy to see.

Confused yet? Hattie’s breakdown of effect size leaves a lot to be desired. The sad part so far is that his research attempts to quantify a large amount of achievement differences and compare the results of a large list of strategies and methods. Teachers and administrators everywhere simply show the list of methods compared by effect size without actually defining what those effect sizes mean.

I think if we look at the effect size of homework we can use that to further understand what Hattie is trying to get at. That is coming in Part 2 later this week.

All paraphrasing and quoting comes from Visible Learning by John Hattie, 2009. Pages 7-8.

§ 9 Responses to Understanding Effect Size In John Hattie’s Research"

  • PJ says:

    Hattie doesn’t have a basic understanding of statistics or probability. His use of effect size of completely different “interventions” (many of his influences on achievement are not even interventions with a properly measured effect on “achievement”, whatever that means) is utter pseudoscience. You cannot compare the effect sizes of different interventions on different dimensions of “achievement”, reasoning that unitless measures are equivalent. Dividing a mean by a standard deviation, because those are on the same scale, makes the unit of measurement disappear, but it does not make the original measured variable vanish from the analysis. There’s a reason the medical field doesn’t do meta-meta-analyses as Hattie attempts to do. Medical researchers collaborate with numerous (bio)statisticians to make sure their analyses make some sense. Hattie hasn’t consulted with any statistician, and the whole field of education seems to have failed to take a single course in basic statistics.

    • matt says:

      Thank you for your in-depth analysis. Although I will try to remain as objective as possible (is it ever truly possible?) I can’t help but feel like he is continually trying to shove many different shaped pegs into one circular hole.

  • […] 2 A continuation from Part 1. As I study John Hattie’s comparison of the effect size of direct instruction vs inquiry […]

    • Midge says:

      It’s much easier to untrnsdaed when you put it that way!

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