How many mailouts? Could attempts to increase the response rate in the Iraq war cohort study be counterproductive?
BACKGROUND
Low response and reporting errors are major concerns for survey epidemiologists. However, while nonresponse is commonly investigated, the effects of misclassification are often ignored, possibly because they are hard to quantify. We investigate both sources of bias in a recent study of the effects of deployment to the 2003 Iraq war on the health of UK military personnel, and attempt to determine whether improving response rates by multiple mailouts was associated with increased misclassification error and hence increased bias in the results.


METHODS
Data for 17,162 UK military personnel were used to determine factors related to response and inverse probability weights were used to assess nonresponse bias. The percentages of inconsistent and missing answers to health questions from the 10,234 responders were used as measures of misclassification in a simulation of the 'true' relative risks that would have been observed if misclassification had not been present. Simulated and observed relative risks of multiple physical symptoms and post-traumatic stress disorder (PTSD) were compared across response waves (number of contact attempts).


RESULTS
Age, rank, gender, ethnic group, enlistment type (regular/reservist) and contact address (military or civilian), but not fitness, were significantly related to response. Weighting for nonresponse had little effect on the relative risks. Of the respondents, 88% had responded by wave 2. Missing answers (total 3%) increased significantly (p < 0.001) between waves 1 and 4 from 2.4% to 7.3%, and the percentage with discrepant answers (total 14%) increased from 12.8% to 16.3% (p = 0.007). However, the adjusted relative risks decreased only slightly from 1.24 to 1.22 for multiple physical symptoms and from 1.12 to 1.09 for PTSD, and showed a similar pattern to those simulated.


CONCLUSIONS
Bias due to nonresponse appears to be small in this study, and increasing the response rates had little effect on the results. Although misclassification is difficult to assess, the results suggest that bias due to reporting errors could be greater than bias caused by nonresponse. Resources might be better spent on improving and validating the data, rather than on increasing the response rate.
King's Centre for Military Health, Research, Institute of Psychiatry, King's College London, London, UK. rosemary.tate@iop.kcl.ac.uk
A Rosemary
AR
Tate
Margaret
M
Jones
Lisa
L
Hull
Nicola T
NT
Fear
Roberto
R
Rona
Simon
S
Wessely
Matthew
M
Hotopf
D000328:Adult; D015982:Bias; D015331:Cohort Studies; D003198:Computer Simulation; D005260:Female; D006801:Humans; D007493:Iraq; D008297:Male; D008875:Middle Aged; D008887:Military Medicine; D008889:Military Personnel / Q000523:psychology / Q000706:statistics & numerical data*; D010809:Physical Fitness; D011159:Population Surveillance / Q000379:methods*; D011177:Postal Service / Q000706:statistics & numerical data*; D012680:Sensitivity and Specificity; D011795:Surveys and Questionnaires / Q000592:standards / Q000706:statistics & numerical data*; D006113:United Kingdom / Q000453:epidemiology