Mike Hull writes:
I was reading over this JAMA Brief Report and could not figure out what they were doing with the composite score. Here are the cliff notes:
Study tested milk vs dark chocolate consumption on three eyesight performance parameters:
(1) High-contrast visual acuity
(2) Small-letter contrast sensitivity
(3) Large-letter contrast sensitivity
Only small-letter contrast sensitivity was significant, but then the authors do this:
Visual acuity was expressed as log of the minimum angle of resolution (logMAR) and contrast sensitivity as the log of the inverse of the minimum detectable contrast (logCS). We scored logMAR and logCS as the number of letters read correctly (0.02 logMAR per visual acuity letter and 0.05 logCS per letter).
Because all 3 measures of spatial vision showed improvement after consumption of dark chocolate, we sought to combine these data in a unique and meaningful way that encompassed different contrasts and letter sizes (spatial frequencies). To quantify overall improvement in spatial vision, we computed the sum of logMAR (corrected for sign) and logCS values from each participant to achieve a composite score that spans letter size and contrast. Composite score results were analyzed using Bland-Altman analysis, with P < .05 indicating significance.
There are more details in the short Results section, but the conclusion was that “Twenty-four participants (80%) showed some improvement with dark chocolate vs milk chocolate (Wilcoxon signed-rank test, P < .001)." Any idea what's going on here? Trial pre-registration here.
I replied that I don’t have much to say on this one. They seemed to have scoured through their data so I’m not surprised they found low p-values. Too bad to see this sort of thing appearing in Jama. I guess chocolate’s such a fun topic, there’s always room for another claim to be made for it.