Tate A Rosemary, Jones Margaret, Hull Lisa, Fear Nicola T, Rona Roberto, Wessely Simon, Hotopf Matthew
King's Centre for Military Health, Research, Institute of Psychiatry, King's College London, London, UK.
BMC Med Res Methodol. 2007 Nov 28;7:51. doi: 10.1186/1471-2288-7-51.
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.
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).
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.
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.
低应答率和报告错误是调查流行病学家主要关注的问题。然而，虽然无应答情况通常会被调查，但错误分类的影响却常常被忽视，这可能是因为它们难以量化。在最近一项关于2003年伊拉克战争部署对英国军事人员健康影响的研究中，我们调查了这两种偏差来源，并试图确定通过多次邮寄提高应答率是否与错误分类误差增加相关，从而导致结果偏差增大。
使用17162名英国军事人员的数据来确定与应答相关的因素，并使用逆概率权重来评估无应答偏差。在模拟如果不存在错误分类时本可观察到的“真实”相对风险时，将10234名应答者对健康问题不一致和缺失答案的百分比用作错误分类的衡量指标。比较了各应答波次（联系尝试次数）中多种身体症状和创伤后应激障碍（PTSD）的模拟相对风险与观察到的相对风险。
年龄、军衔、性别、种族、入伍类型（正规军/预备役）和联系地址（军事或民用），而非健康状况，与应答显著相关。对无应答进行加权对相对风险影响不大。在应答者中，88%在第2波次时做出了应答。缺失答案（总计3%）在第1波次和第4波次之间从2.4%显著增加至7.3%（p < 0.001），答案不一致的百分比（总计14%）从12.8%增加至16.3%（p = 0.007）。然而，对于多种身体症状，调整后的相对风险仅从1.24略微降至1.22，对于PTSD从1.12降至1.09，并且呈现出与模拟结果相似的模式。
在本研究中，无应答导致的偏差似乎较小，提高应答率对结果影响不大。虽然错误分类难以评估，但结果表明报告错误导致的偏差可能大于无应答导致的偏差。资源或许应更好地用于改进和验证数据，而非提高应答率。
