Additional file 1.
Bland-Altman plot for the difference in caffeine intake between the food frequency questionnaire (FFQ) and a four-day weighed food diary in 119 women in the validation study. Bland-Altman plot of the differences in caffeine intake between the FFQ and the food diary measurements (bias) against the mean caffeine intake by the two methods showing that the mean difference was small and not biased towards any of the methods. The median (IQR) caffeine intake in the validation study sample was 40 mg/day (18 to 88 mg/day) by the FFQ and 38 mg/day (10 to 99 mg/day) by the food diary. Spearman correlation was 0.70 (95% CI 0.59 to 0.78).
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Sengpiel et al. BMC Medicine 2013 11:42 doi:10.1186/1741-7015-11-42