Lifetime body size and reproductive factors: comparisons of data recorded prospectively with self reports in middle age

Abstract

Background: Data on lifetime exposures are often self-reported in epidemiologic studies, sometimes many years after the relevant age. Validity of self-reported data is usually inferred from their agreement with measured values, but few studies directly quantify the likely effects of reporting errors in body size and reproductive history variables on estimates of disease-exposure associations.

Methods: The MRC National Survey of Health and Development (NSHD) and the Million Women Study (MWS) are UK population-based prospective cohorts. The NSHD recruited participants at birth in 1946 and has followed them at regular intervals since then, whereas the MWS recruited women in middle age. For 541 women who were participants in both studies, we used statistical measures of association and agreement to compare self-reported MWS data on body size throughout life and reproductive history, obtained in middle age, to NSHD data measured or reported close to the relevant ages. Likely attenuation of estimates of linear disease-exposure associations due to the combined effects of random and systematic errors was quantified using regression dilution ratios (RDRs).

Results: Data from the two studies were very strongly correlated for current height, weight and body mass index, and age at menopause (Pearson r = 0.91-0.95), strongly correlated for birth weight, parental heights, current waist and hip circumferences and waist-to-height ratio (r = 0.67-0.80), and moderately correlated for age at menarche and waist-to-hip ratio (r = 0.52-0.57). Self-reported categorical body size and clothes size data for various ages were moderately to strongly associated with anthropometry collected at the relevant times (Spearman correlations 0.51-0.79). Overall agreement between the studies was also good for most quantitative variables, although all exhibited both random and systematic reporting error. RDRs ranged from 0.66 to 0.86 for most variables (slight to moderate attenuation), except weight and body mass index (1.02 and 1.04, respectively; little or no attenuation), and age at menarche, birth weight and waist-to-hip ratio (0.44, 0.59 and 0.50, respectively; substantial attenuation).

Conclusions: This study provides some evidence that self-reported data on certain anthropometric and reproductive factors may be adequate for describing disease-exposure associations in large epidemiological studies, provided that the effects of reporting errors are quantified and the results are interpreted with caution.

Figure 1

Quantification of the effects of reporting errors in MWS anthropometry and reproductive history variables. Means and 95% confidence intervals for NSHD variables are plotted against means of corresponding MWS variables, within selected categories of the MWS data. Category boundaries are given on the horizontal axes. Pearson correlation coefficients (r) are indicated for each variable, as are regression dilution ratios (RDR) with 95% bootstrapped confidence bounds, indicating the likely relative attenuation of linear coefficients for disease-exposure associations. The dashed lines are lines of equality of means.

Figure 2

Comparisons of NSHD anthropometry against corresponding ordinal MWS body size variables at various ages. Means and 95% confidence intervals of NSHD anthropometric variables are plotted according to categories of MWS body size variables. (A) NSHD BMI measured at age 11 versus MWS relative body size at age 10; (B) NSHD BMI reported at age 20 versus MWS clothes size at age 20; (C) NSHD BMI measured at age 53 versus MWS clothes size in middle age; (D) NSHD waist circumference measured at age 53 versus MWS clothes size in middle age; (E) NSHD chest circumference measured at age 53 versus MWS bra band size in middle age. Spearman correlations (r) are indicated for each pair of NSHD and MWS variables.