J R Penrod1, P M Lantz. 1. Faculty of Medicine, McGill University/Montreal General Hospital, QC, Canada. mdpn@musica.mcgill.ca
Abstract
OBJECTIVE: Errors in the measurement of the timing and number of prenatal care visits may produce downward bias in estimates of the impact of prenatal care use on birth outcomes. This paper examines the extent of attenuation bias from measurement error in the estimation of the effect of prenatal care use on birth weight. METHODS: Data were analyzed from the 1980 National Natality Survey, a nationally representative sample of live births with information on prenatal care utilization from three sources: birth certificates, medical provider surveys, and maternal surveys. The extent of attenuation bias in estimates of the impact of different measures of prenatal care use on birth weight was examined by comparing estimates robust to measurement error (including instrumental variables) with ordinary least squares results. RESULTS: There is considerable disagreement in measures of prenatal care across the three data sources, with correlations in the utilization measures computed from different sources around 0.5. The results also show evidence of attenuation bias from measurement error in estimates of the impact of prenatal care on birth weight for both White and Black mothers. Attenuation bias was least severe for information from the birth certificate report of prenatal care. CONCLUSIONS: Because of measurement error, previous studies may have underestimated the effect of prenatal care utilization on birth weight. Corrected estimates, however, do not suggest that prenatal care is a major predictor of birth weight. In addition, part of what previous analyses have interpreted as adverse selection bias may in fact be attenuation bias due to measurement error.
OBJECTIVE: Errors in the measurement of the timing and number of prenatal care visits may produce downward bias in estimates of the impact of prenatal care use on birth outcomes. This paper examines the extent of attenuation bias from measurement error in the estimation of the effect of prenatal care use on birth weight. METHODS: Data were analyzed from the 1980 National Natality Survey, a nationally representative sample of live births with information on prenatal care utilization from three sources: birth certificates, medical provider surveys, and maternal surveys. The extent of attenuation bias in estimates of the impact of different measures of prenatal care use on birth weight was examined by comparing estimates robust to measurement error (including instrumental variables) with ordinary least squares results. RESULTS: There is considerable disagreement in measures of prenatal care across the three data sources, with correlations in the utilization measures computed from different sources around 0.5. The results also show evidence of attenuation bias from measurement error in estimates of the impact of prenatal care on birth weight for both White and Black mothers. Attenuation bias was least severe for information from the birth certificate report of prenatal care. CONCLUSIONS: Because of measurement error, previous studies may have underestimated the effect of prenatal care utilization on birth weight. Corrected estimates, however, do not suggest that prenatal care is a major predictor of birth weight. In addition, part of what previous analyses have interpreted as adverse selection bias may in fact be attenuation bias due to measurement error.