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Ann Epidemiol. 2005 Aug;15(7):509-18.

Correlations between estimated and true dietary intakes: using two instrumental variables.

Author information

1
Department of Epidemiology and Biostatistics, Loma Linda University, Loma Linda, CA 92313, USA. gfraser@sph.llu.edu

Abstract

PURPOSE:

We describe a new application of the method of triads that allows an estimate of the correlation between a dietary questionnaire measure (Q) and true intake (T).

METHODS:

Three surrogate variables Q, M, and P are observed where M and P are both instrumental (often biological) variables. A reference dietary method (R) is not required. The variables M and P may be concentration rather than recovery biomarkers. Estimating equations produce Corr(Q,T), Corr(M,T), Corr(P,T), conditional on assumptions about error correlations. Correlations between errors in both Q and a reference dietary measure can also be estimated if R is available. A small validation study of California Seventh-day Adventists provided food frequency, repeated 24-hour dietary recalls (R), and biological data (blood, overnight urines, and subcutaneous fat).

RESULTS:

Values of Corr(Q,T) ranged between 0.40 and 0.66. Values of Corr(R,T) were higher, between 0.48 and 0.83. Estimated correlations between errors in R and Q were all positive.

CONCLUSIONS:

When carefully chosen, M and P, rather than M and R, should better satisfy assumptions about error correlations. Food frequency data and repeated 24-hour recalls both provide estimates of T, but the latter has greater validity. Standard errors suggest that for good precision Corr(Q,T) requires large validation studies (2000-3000 subjects).

PMID:
16029843
DOI:
10.1016/j.annepidem.2004.12.012
[Indexed for MEDLINE]

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