THE MILLION WOMEN STUDY

HomeMillion Women Study

Return to publication list

Validity over time of self-reported anthropometric variables during follow-up of a large cohort of UK women.

Wright FL, Green J, Reeves G, Beral V, Cairns BJ; Million Women Study collaborators.

BMC Med Res Methodol 2015;15:81.

BACKGROUND:
In prospective epidemiological studies, anthropometry is often self-reported and may be subject to reporting errors. Self-reported anthropometric data are reasonably accurate when compared with measurements made at the same time, but reporting errors and changes over time in anthropometric characteristics could potentially generate time-dependent biases in disease-exposure associations.

METHODS:
In a sample of about 4000 middle-aged UK women from a large prospective cohort study, we compared repeated self-reports of weight, height, derived body mass index, and waist and hip circumferences, obtained between 1999 and 2008, with clinical measurements taken in 2008. For self-reported and measured values of each variable, mean differences, correlation coefficients, and regression dilution ratios (which measure relative bias in estimates of linear association) were compared over time.

RESULTS:
For most variables, the differences between self-reported and measured values were small. On average, reported values tended to be lower than measured values (i.e. under-reported) for all variables except height; under-reporting was greatest for waist circumference. As expected, the greater the elapsed time between self-report and measurement, the larger the mean differences between them (each Pā€‰<ā€‰0.001 for trend), and the weaker their correlations (each Pā€‰<ā€‰0.004 for trend). Regression dilution ratios were in general close to 1.0 and did not vary greatly over time.

CONCLUSION:
Reporting errors in anthropometric variables may result in small biases to estimates of associations with disease outcomes. Weaker correlations between self-reported and measured values would result in some loss of study power over time. Overall, however, our results provide new evidence that self-reported anthropometric variables remain suitable for use in analyses of associations with disease outcomes in cohort studies over at least a decade of follow-up.

---