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Glossary of Epidemiology: 5) Exposure - Measurement

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Major problems with measurement
Information bias.
prospective studies - nondifferential
When E is measured after D - diffential
Evaluating measurement methods
biological methods.
subject interview.
records.
6 Measuring factors
1)Accuracy and complete
2)Nondifferential
3)Response rate
4)Temporal tracking
5)Availability
6)Cost
Current exposure: Biological or physical measurement
often gold std.
Current:
Accuracy and complete ++
nondifferential ++
response rate -
temproal tracking +
availability +
cost -
past exposure: biological or physical measurement
Past
Accuracy and complete --
nondifferential --
response rate --
temproal tracking --
availability --
cost --
Coefficient of variation
variation expresses as % of the mean
Interview
most common method but is problematic for past exposures or exposure after disease
most common problems of interviews
poor technique.
fail to cross-check with other data sources.
insensitive to counterincentives to accurate reporting
Current exposure: Interview
Current:
Accuracy and complete ?
nondifferential +
response rate +
temproal tracking +
availability ++
cost +
Past exposure: Interview
Past
Accuracy and complete -
nondifferential -
response rate +
temproal tracking --
availability +
cost +
Current exposure: records
Current:
Accuracy and complete ?
nondifferential ++
response rate ++
temproal tracking ++
availability -
cost ++
Past exposure: records
Past:
Accuracy and complete ?
nondifferential ++
response rate ++
temporal tracking ++
availability -
cost ++
Assessing measurment quality
Requires gold standard
1)Categorical: Sensitivity, Specificity, PPV
2)Product moment correlation coefficient (Pearson)
Indices of reproducibility
Categorical: Kappa
Continuous: Intra-class correlation
Effects of nondifferential measurement error
Bias towards Null
Effect of differential exposure misclassification
Spurious diffences between E+ and E-
Exposure misclassification in a cohort study
Baseline measurement.
Unmeasured changes in exposure during followup
Exposure misclassification in a case-control study
1)Recall failure (non-differential)
2)Differential recall between cases and controls
Minimizing exposure misclassification
1)Good technique: Standardized, structureed questions, repeate measures.
2)Use of records
3)Substudy estimate misclassification, mathematical correlation

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