Effect Size d= 0.12  (Hattie's Rank=123)

One meta-analysis was used:
Karvale, K. & Forness,S., (1983) Hyperactivity and diet treatment. Journal of Learning Disabilities. 16(6), 324-330.

This paper is not measuring diet as it relates to improving student achievement. But rather diet modification as a treatment of hyperactivity. In addition, the research is only about a particular type of diet modification based on the hypothesis that, "the ingestion of artificial (synthetic) food additives (colours and flavours) and naturally occurring salicylates in foods results in hyperactivity and learning disabilities in children... It was suggested that treatment be based on the Feingold Kaiser-Permanente (K-P) diet which is designed to eliminate all foods containing natural salicylates and artificial food additives from the diet" (p324).

Hyperactivity was measured by a number of different tests: e.g., Connors' Scale parents, Connors' Scale teachers, attention, disruptive behaviour, impulsiveness, global improvement, learning ability and hyperkinesis. The researchers report (p327) a summary of effect sizes as listed below:

Hattie seems to have averaged all the categories except for 'Impulsivity' and reports an average, d = 0.12. There is no explanation about this in his commentary on the diet on pages 52-53 of VL.

So once again Hattie includes items that are not measuring achievement. So significant doubts are raised about the validity of this analysis in the context of VL - i.e., improving student achievement.

From docendo:
"There is one meta-analysis, Kavale and Forness (1983). I can only access the abstract but it’s clear that despite the missing clause in Hattie’s summary, the meaning that I had assumed he intended does match this meta-analysis. Equally, it is clear that this is very specifically looking at ADHD and not children without this diagnosis. Essentially this paper states that the studies analysed do not provide evidence to support the earlier hypothesis that dietary changes could have a positive effect on ADHD symptoms. I’m guessing that the outcome measure was not academic achievement, but more likely some behavioural measure, which reminds me again that Hattie seems rather blasé about what his meta-analyses are measuring."