EFFECT OF MEASUREMENT ERROR ON EPIDEMIOLOGIC STUDIES OF ENVIRONMENTALAND OCCUPATIONAL EXPOSURES

Random error (misclassification) in exposure measurements usually bias es a relative risk, regression coefficient, or other effect measure to wards the null value (no association). The most important exception is Berkson type error, which causes little or no bias. Berkson type erro r arises, in particular, due to use of group average exposure in place of individual values. Random error in exposure measurements, Berkson or otherwise, reduces the power of a study, making it more likely that real associations are not detected. Random error in confounding varia bles compromises the control of their effect, leaving residual confoun ding. Random error in a variable that modifies the effect of exposure on health-for example, an indicator of susceptibility-tends to diminis h the observed modification of effect, but error in the exposure can c reate a spurious appearance of modification. Methods are available to correct for bias (but not generally power loss) due to measurement err or, if information on the magnitude and type of error is available. Th ese methods can be complicated to use, however, and should be used cau tiously as ''correction'' can magnify confounding if it is present.