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Facts Sheet for ProUCL 4.0 A Statistical Software Package for Environmental Applications for Data Sets With and Without Nondetect Observations Exposure assessment,risk assessment and management, and cleanup decisions at potentially polluted sites are often made based upon the mean concentrations of the contaminants of potential concern(COPCs). Typically,the mean concentration of a COPC at a contaminated site is unknown, and is frequently estimated by the sample mean based upon the data collected from the site areas under investigation. In order to address the uncertainties associated with the estimates of the unknown mean concentrations of the COPCs, appropriate 95%upper confidence limits (UCLs) of the respective unknown means are used in many environmental applications including the estimation of exposure point concentration(EPC)terms. The Technical Support Center(TSC),EPA Las Vegas,NV developed ProUCL Version 3.0 software package (EPA, 2004)to support risk assessment and cleanup decisions at contaminated sites based upon full data sets without nondetect observations (NDs). For data sets without NDs, ProUCL 3.0 has several parametric and nonparametric UCL computation methods as described in the revised EPA UCL Guidance Document for Hazardous Waste Sites(EPA, 2002a). The Need for an Upgrade of ProUCL 3.0 Nondetect observations are inevitable in most data sets collected from the various environmental applications. The ProUCL 4.0 software package is an upgrade of ProUCL 3.0 software package, and provides several statistical methods that can be used on left censored data sets with nondetect observations potentially having multiple detection limits (DLs). ProUCL 4.0 is especially developed to address the various statistical issues arising in exposure and risk assessment studies, and also in background and site evaluation and comparison applications. All capabilities of ProUCL 3.0 have been retained in ProUCL 4.0. The TSC,EPA Las Vegas has revised the Background Guidance Document for CERCLA sites (EPA, 2002b). The revised background document includes some exploratory graphical methods to pre-process a data set, and a couple of new chapters describing the computations of parametric and nonparametric upper limits that are used to estimate the background level contaminant concentrations or background threshold values (BTVs), and other not-to-exceed values based upon data sets with and without ND observations. It is noted that the methods to compute upper limits to estimate BTVs and not-to-exceed values are not easily available in any of the available software packages, especially for data sets with nondetect observations. ProUCL 4.0 can be used to compute various parametric and nonparametric upper limits often used to estimate environmental parameters of interest including the EPC terms, BTVs, and other not-to-exceed values. The BTVs and not-to-exceed values are also used for screening of contaminants of potential concern(COPCs). Typically,upper confidence limits (UCLs)are used to estimate the EPC terms; upper prediction limits (UPLs), upper tolerance limits (UTLs), or upper percentiles are used to estimate the BTVs and not-to- exceed values. ProUCL 4.0 can be used to compute those upper limits based upon full uncensored data sets without NDs and left-censored data sets with NDs having multiple DLs. Additionally, ProUCL 4.0 offers several parametric and nonparametric single sample and two sample hypotheses testing approaches used in background versus site comparison studies. Those hypotheses testing approaches can be used on data sets with NDs and without NDs. ProUCL 4.