Proxy respondents in a case-control study: validity, reliability and impact
Tomkins, Susannah Chloe;
(2006)
Proxy respondents in a case-control study: validity, reliability and impact.
PhD thesis, London School of Hygiene & Tropical Medicine.
DOI: https://doi.org/10.17037/PUBS.00768483
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In many research settings, the study subject is unable to provide responses, requiring
researchers to find alternative respondents. This thesis explores the use of proxy
respondents in a case control study of premature mortality among Russian men of
working age (25-54 years).
Data obtained from proxy respondents is explored in four ways. Firstly, proxy
questionnaire responses are validated against external data sources which were
routinely collected, blind to case-control status - the city alcohol treatment clinic
(Narcology Dispensary), Social Security and Police records. Secondly, agreement
between proxy and index (control) responses to questions about alcohol use, tobacco
use, health and socioeconomic factors is explored. Thirdly, the effect of proxy type is
explored by examination of proxy-proxy and index-proxy agreement in a subset of
households in which two proxy interviews were obtained. Finally, the impact on
analysis outcomes is explored by mortality analyses using proxy versus index data.
Cohen's kappa coefficient was used to explore inter-respondent agreement.
Differences in agreement between pairs of respondents were examined using
Agresti's loglinear model, and the directionality of disagreements were evaluated
using McNemar's test.
Findings confirm some assertions in the literature. Questions about easily observable
characteristics and behaviours, avoiding excessive detail, subjective or sensitive
topics, elicit valid proxy responses. Proxies tend to over-report alcohol use, but
provide particularly valid responses about tobacco use and socioeconomic factors.
Validity was highest among proxies who were the index's spouse. However, further
exploration suggested that men who have spouses differ in their behaviour from men
who do not in ways which affect its reporting. There was little additional evidence
that proxy characteristics affect validity. The use of proxy responses biases odds
ratios in this case control study toward more conservative estimates. These findings
are generalisable to study settings which employ a protocol to ensure selection of the
best available proxy.