A Macro for Calculating Summary Statistics on Left Censored Environmental Data using the Kaplan-Meier Method

Calculating summary statistics such as the mean, standard deviation, and an upper confidence limit on the mean is straightforward when the data values are known. However, environmental data often are reported from the analytical laboratory as left censored, meaning the actual concentration for a given contaminant was not detected above the method detection limit. Therefore, the true concentration is known only to be between 0 and the reporting limit. The nonparametric Kaplan-Meier product limit estimator has been widely used in survival analysis on right censored data, but only recently has this method been applied to left censored data. Kaplan-Meier can be used on censored data with multiple reporting limits with minimal assumptions. This paper presents a SAS macro that calculates the mean, standard deviation, and standard error of the mean of a left censored environmental data set using the nonparametric Kaplan-Meier method. Kaplan-Meier has been shown to provide more robust estimates of the mean and standard deviation of left censored data than other methods such as simple substitution and maximum likelihood estimates. This paper is for intermediate SAS users of SAS/BASE.