Variability assessment
We assessed variability in BDCPP and DPP in three ways. First, we estimated the short- and long-term variability in BDCPP and DPP concentrations by calculating intraclass correlation coefficients (ICCs). ICCs provide a measure of the reliability of repeated measures over time and are calculated by taking the ratio of the between-subject variability to the sum of the between- and within-subject variability (Rosner, 2000). ICC values range from 0, indicating no reproducibility, to 1, indicating perfect reproducibility. ICCs and 95% confidence intervals were calculated using a SAS Marco developed by Hamer (1995), and based on the work of Shrout and Fleiss (1979). ICCs were calculated for samples collected in the 18th week of pregnancy to capture short-term reliability (3 samples each in 8 women; n = 24) and for all sample points to capture long-term reliability. In analyses of long-term variability, the three samples collected from each woman during the 18th week of pregnancy were first averaged to avoid inflating ICCs with measurements taken over a short period of time, and then compared to the samples taken at 28th week of pregnancy and at birth.
In addition to ICCs, we also conducted analyses to determine how well OPFR metabolite concentrations from each time point (18 week average, 28th week, and birth samples) captured the rank order of exposure throughout pregnancy. First, each woman's geometric mean metabolite concentration was calculated and women were ranked (GM of 18 week average, 28th week, and birth samples). Women with the 4 highest GM concentrations were classified as the “high” exposure group (true high exposure category). Women were then ranked at each time point based on the concentration in that individual sample (predicted exposure category). Contingency tables were constructed for each time point comparing the predicted exposure category to the true high exposure category. Contingency tables form each time point were then combined into a single table and sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated.
Variability assessment
We assessed variability in BDCPP and DPP in three ways. First, we estimated the short- and long-term variability in BDCPP and DPP concentrations by calculating intraclass correlation coefficients (ICCs). ICCs provide a measure of the reliability of repeated measures over time and are calculated by taking the ratio of the between-subject variability to the sum of the between- and within-subject variability (Rosner, 2000). ICC values range from 0, indicating no reproducibility, to 1, indicating perfect reproducibility. ICCs and 95% confidence intervals were calculated using a SAS Marco developed by Hamer (1995), and based on the work of Shrout and Fleiss (1979). ICCs were calculated for samples collected in the 18th week of pregnancy to capture short-term reliability (3 samples each in 8 women; n = 24) and for all sample points to capture long-term reliability. In analyses of long-term variability, the three samples collected from each woman during the 18th week of pregnancy were first averaged to avoid inflating ICCs with measurements taken over a short period of time, and then compared to the samples taken at 28th week of pregnancy and at birth.
In addition to ICCs, we also conducted analyses to determine how well OPFR metabolite concentrations from each time point (18 week average, 28th week, and birth samples) captured the rank order of exposure throughout pregnancy. First, each woman's geometric mean metabolite concentration was calculated and women were ranked (GM of 18 week average, 28th week, and birth samples). Women with the 4 highest GM concentrations were classified as the “high” exposure group (true high exposure category). Women were then ranked at each time point based on the concentration in that individual sample (predicted exposure category). Contingency tables were constructed for each time point comparing the predicted exposure category to the true high exposure category. Contingency tables form each time point were then combined into a single table and sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated.
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