title: Dietary assessment in the California Teachers Study: reproducibility and validity.
abstract: OBJECTIVE
  To evaluate the reproducibility and validity of the food-frequency questionnaire (FFQ) used in the California Teachers Study (CTS) cohort and to use this data to quantify the effects of correcting nutrient-breast cancer relative risks for measurement error.
  
  
  METHODS
  One hundred and ninety five CTS cohort members participated in a 10-month dietary validation study that included four 24-h dietary recalls and pre- and post-study FFQs. Shrout-Fleiss intraclass correlations for reproducibility were computed. Under several standard assumptions concerning the correlations of errors in the FFQs and 24-h recalls, we calculated energy-adjusted deattenuated Pearson correlations for validity and tested for differences in validity according to a number of demographic and other risk factors. For each nutrient, we compared the performance of the FFQ versus the 24-h recalls, estimating the number of days of recalls that give equivalent information about true intake as does a single FFQ. Finally, the effects of adjustment for measurement error on risk estimates were evaluated in 44,423 postmenopausal cohort members, 1,544 of whom developed breast cancer during seven years of follow-up. Relative risks (RR) and confidence intervals (CI) were calculated using Cox proportional hazards with and without correction for measurement error.
  
  
  RESULTS
  Reproducibility correlations for the nutrients ranged from 0.60 to 0.87. With a few exceptions, validity correlations were reasonably high (range: 0.55-0.85), including r = 0.74 for alcohol. Performance of the FFQ differed by age for percent of calories from fat and by body mass index and hormone therapy use for alcohol consumption. For most nutrients examined, our FFQ is comparable to two to six recalls for each subject in capturing true intake. In the measurement error-adjusted risk analyses, corrected RRs were within 13% of uncorrected values for all nutrients examined except for linoleic acid. For alcohol consumption the corrected RR (per 20 g/1,000 kcal/d) was 1.36 (95% CI: 1.03-1.51) compared to the uncorrected estimate of 1.25 (95% CI: 1.10-1.42).
  
  
  CONCLUSIONS
  The FFQ dietary assessment used in the CTS is reproducible and valid for all nutrients except the unsaturated fatty acids. Correcting relative risk estimates for measurement error resulted in relatively small changes in the associations between the majority of nutrients and the risk of postmenopausal breast cancer.
authors.affiliation: Northern California Cancer Center, 2201 Walnut Avenue, Fremont, CA 94538, USA. pam@nccc.org
authors.forename: Pamela L
authors.initials: PL
authors.lastname: Horn-Ross
authors.forename: Valerie S
authors.initials: VS
authors.lastname: Lee
authors.forename: Christine N
authors.initials: CN
authors.lastname: Collins
authors.forename: Susan L
authors.initials: SL
authors.lastname: Stewart
authors.forename: Alison J
authors.initials: AJ
authors.lastname: Canchola
authors.forename: Marion M
authors.initials: MM
authors.lastname: Lee
authors.forename: Peggy
authors.initials: P
authors.lastname: Reynolds
authors.forename: Christina A
authors.initials: CA
authors.lastname: Clarke
authors.forename: Leslie
authors.initials: L
authors.lastname: Bernstein
authors.forename: Daniel O
authors.initials: DO
authors.lastname: Stram
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000428:Alcohol Drinking / Q000009:adverse effects; D001943:Breast Neoplasms / Q000209:etiology*; D002140:California; D004034:Diet Surveys; D005178:Faculty; D005247:Feeding Behavior; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008875:Middle Aged; D015203:Reproducibility of Results; D012306:Risk; D011795:Surveys and Questionnaires