title: Determination of confidence intervals in non-normal data: application of the bootstrap to cocaine concentration in femoral blood.
abstract: Calculating the confidence interval is a common procedure in data analysis and is readily obtained from normally distributed populations with the familiar [Formula: see text] formula. However, when working with non-normally distributed data, determining the confidence interval is not as obvious. For this type of data, there are fewer references in the literature, and they are much less accessible. We describe, in simple language, the percentile and bias-corrected and accelerated variations of the bootstrap method to calculate confidence intervals. This method can be applied to a wide variety of parameters (mean, median, slope of a calibration curve, etc.) and is appropriate for normal and non-normal data sets. As a worked example, the confidence interval around the median concentration of cocaine in femoral blood is calculated using bootstrap techniques. The median of the non-toxic concentrations was 46.7 ng/mL with a 95% confidence interval of 23.9-85.8 ng/mL in the non-normally distributed set of 45 postmortem cases. This method should be used to lead to more statistically sound and accurate confidence intervals for non-normally distributed populations, such as reference values of therapeutic and toxic drug concentration, as well as situations of truncated concentration values near the limit of quantification or cutoff of a method.
authors.affiliation: Department of Toxicology, Laboratoire de Sciences Judiciaires et de Médecine Légale, 1701 rue Parthenais, Montréal, QC H2K 3S7, Canada Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montréal, QC H4B 1R6, Canada brigitte.desharnais@msp.gouv.qc.ca.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Desharnais
authors.affiliation: Department of Mathematics, Université de Sherbrooke, 2500 Boulevard de L'Université, Sherbrooke, QC J1K 2R1, Canada.
authors.forename: Félix
authors.initials: F
authors.lastname: Camirand-Lemyre
authors.affiliation: Department of Toxicology, Laboratoire de Sciences Judiciaires et de Médecine Légale, 1701 rue Parthenais, Montréal, QC H2K 3S7, Canada.
authors.forename: Pascal
authors.initials: P
authors.lastname: Mireault
authors.affiliation: Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montréal, QC H4B 1R6, Canada.
authors.forename: Cameron D
authors.initials: CD
authors.lastname: Skinner
mesh_terms: D003042:Cocaine / Q000097:blood*; D016001:Confidence Intervals; D003627:Data Interpretation, Statistical; D005268:Femoral Vein; D006801:Humans
chemical_list: D003042:Cocaine