Table 2 also includes the results of linear model selection using the LASSO technique, which eliminates covariates with unstable estimates and results in more parsimonious models. Because the LASSO method uses bootstrapping to obtain standard errors, the coefficient of any covariate may be shrunk to zero if that covariate is an unstable predictor— that is, if its significance depends on the par- ticular subset of data used in the model. The two approaches yielded very similar estimates of CPF effect. Differences in estimates for the covariates in the two methods suggest that the contribution of some covariates to WISC-IV scores may be less stable. Results for both approaches show that, on average, a 1-pg/g increase in CPF is associated with a decrease of –0.006 points in the log-transformed Working Memory score and a decrease of –0.003 points in the log-transformed Full- Scale IQ score. Because of the log transforma- tion, estimated associations between CPF and actual Working Memory and Full-Scale IQ scores vary across the continuum of scores, such that the estimated deficit in the Working Memory score with a 1-pg/g increase in CPF ranges between 0.35 and 0.81 points, and the estimated decrease in Full-Scale IQ is between 0.20 and 0.40 points. The magni- tude of these effects is more easily understood by calculating the neurodevelopmental deficit associated with an increase in CPF exposure equal to 1 SD (4.61 pg/g). On average, for each standard deviation increase in exposure, Full-Scale IQ declines by 1.4% and Working Memory declines by 2.8%. We found no sig- nificant interactions between CPF and any of the potential or final covariates, including the other chemical exposures measured dur- ing the prenatal period (ETS and PAH). Full model results for the linear regressions are provided in Supplemental Material, Table 1 (doi:10.1289/ehp.1003160), for the reader who is interested in the estimates of associa- tion between the covariates and outcomes for all of the WISC-IV index scales. Sensitivity analysis of additional influ- ences on Working Memory Index. To deter- mine whether the observed CPF effect on the Working Memory Index was partially explained by its effect on general intel- ligence, we added the log-transformed General Ability Index, a general intelligence scale that does not include the Working Memory Index or Processing Speed Index, to the linear regression model. Although the estimate of the General Ability Index effect on Working Memory Index was significant (B-coefficient = 0.57; 95% CI,0.44–0.70; p < 0.001), the estimate of the CPF effect remained unchanged (–0.006), and we found no evidence of interaction between CPF and General Ability Index (p > 0.05), suggesting that the Working Memory effect is targeted 0 5 10 CPF (pg/g) 15 Figure 1. Smoothed cubic splines, superimposed over scatterplots, examining the shape of the asso- ciations between CPF exposure and (A) Working Memory Index, (B) Verbal Comprehension Index, (C) Perceptual Reasoning Index, (D) Processing Speed Index, and (E) Full-Scale IQ. Environmental Health Perspectives • volume 119 | number 8 | August 2011 1199 Neurodevelopment and chlorpyrifos and does not depend upon level of general intelligence. Because child performance on the Working Memory Index can be influenced by child behavior problems (Wechsler 2003), we conducted a supplementary analysis to rule out the possibility that the observed associa- tions between CPF and the Working Memory Index might be affected by behavior prob- lems, as measured by the clinically oriented diagnostic and statistical manual scales on the Child Behavior Checklist. We found no evidence of indirect “mediation” using Sobel’s test, with p-values ranging from 0.31 to 0.99 (MacKinnon et al. 2002; Sobel 1982). Full model results are provided in Supplemental Material, Table 2 (doi:10.1289/ehp.1003160), for the reader who is interested in the esti- mates of association between child behavior problems and Working Memory Index. Sensitivity analysis of the influence of LOD imputation. After obtaining all results, we recomputed all estimates of association between CPF and WISC-IV scores among Working Memory Index Verbal Comprehension Index e ro4.8 c s e its4.7 o p m co4.6 n o i sn4.5 e h e rp4.4 m o C la4.3 b r e gV4.2 o L re4.8 o c s te4.7 i s o pm4.6 o c d ee4.5 p S ing4.4 s s e roc4.3 P g Lo4.2 0 0 5 5 10 CPF (pg/g) 15 Processing Speed Index 10 CPF (pg/g) 15 20 20 e4.8 r o c se4.7 t i s o p4.6 m o c ryo4.5 m e M4.4 g n i k ro4.3 W g Lo4.2 e ro4.8 c s e its4.7 o p m oc4.6 g n ion4.5 s a e lR4.4 a u t ecp4.3 r e P g4.2 o L 4.8 e roc4.7 s e t is4.6 o p m co4.5 e l a -cS4.4 l l u Fg4.3 o L 4.2 0 0 5 5 10 CPF (pg/g) 15 Perceptual Reasoning Index 10 CPF (pg/g) 15 Full-Scale IQ 20 20 20