University of Queensland, School of Population Health, Herston, QLD 4006, Australia.
BMC Med Res Methodol. 2010 Aug 5;10:71. doi: 10.1186/1471-2288-10-71.
There are well-established risk factors, such as lower education, for attrition of study participants. Consequently, the representativeness of the cohort in a longitudinal study may deteriorate over time. Death is a common form of attrition in cohort studies of older people. The aim of this paper is to examine the effects of death and other forms of attrition on risk factor prevalence in the study cohort and the target population over time.
Differential associations between a risk factor and death and non-death attrition are considered under various hypothetical conditions. Empirical data from the Australian Longitudinal Study on Women's Health (ALSWH) for participants born in 1921-26 are used to identify associations which occur in practice, and national cross-sectional data from Australian Censuses and National Health Surveys are used to illustrate the evolution of bias over approximately ten years.
The hypothetical situations illustrate how death and other attrition can theoretically affect changes in bias over time. Between 1996 and 2008, 28.4% of ALSWH participants died, 16.5% withdrew and 10.4% were lost to follow up. There were differential associations with various risk factors, for example, non-English speaking country of birth was associated with non-death attrition but not death whereas being underweight (body mass index < 18.5) was associated with death but not other forms of attrition. Compared to national data, underrepresentation of women with non-English speaking country of birth increased from 3.9% to 7.2% and over-representation of current and ex-smoking increased from 2.6% to 5.8%.
Deaths occur in both the target population and study cohort, while other forms of attrition occur only in the study cohort. Therefore non-death attrition may cause greater bias than death in longitudinal studies. However although more than a quarter of the oldest participants in the ALSWH died in the 12 years following recruitment, differences from the national population changed only slightly.
已有大量研究确定了一些研究参与者流失的风险因素，如受教育程度较低。因此，纵向研究中的队列代表性可能会随时间推移而恶化。在老年人队列研究中，死亡是参与者流失的常见形式。本文旨在研究死亡和其他形式的流失对研究队列和目标人群中风险因素流行率随时间的影响。
在各种假设条件下，考虑了风险因素与死亡和非死亡流失之间的差异关联。使用澳大利亚妇女健康纵向研究（ALSWH）中 1921-1926 年出生的参与者的实证数据，确定了实际存在的关联，并使用澳大利亚人口普查和国家健康调查的全国性横断面数据来说明大约十年内偏差的演变。
假设情况说明了死亡和其他流失如何在理论上影响随时间变化的偏差。在 1996 年至 2008 年间，28.4%的 ALSWH 参与者死亡，16.5%退出，10.4%失访。与各种风险因素存在差异关联，例如，非英语国家出生与非死亡流失相关，而不是死亡；而体重过轻（体重指数<18.5）与死亡相关，而与其他形式的流失无关。与全国数据相比，非英语国家出生的女性代表性不足从 3.9%增加到 7.2%，目前和曾经吸烟的代表性过高从 2.6%增加到 5.8%。
目标人群和研究队列中都有死亡发生，而其他形式的流失仅发生在研究队列中。因此，非死亡流失可能比死亡在纵向研究中产生更大的偏差。然而，尽管在招募后 12 年内，ALSWH 中最年长的参与者中有超过四分之一死亡，但与全国人口的差异仅略有变化。
