D6250-98 – Standard Test Method Technical Guide

🎯 Purpose and Scope of Decision Point Derivation

This practice covers a logical basis for the derivation of a decision point and confidence limit when mean concentration is the parameter used for making environmental waste management decisions. The determination of these values should be made in the context of the defined problem, with the main focus on the derivation of the decision point.

In this framework, the decision point allows a direct comparison of a sample mean against this derived value. A similar decision can be made by comparing a confidence limit against a concentration limit (e.g., a regulatory limit, used as a surrogate term throughout the standard). This practice focuses specifically on environmental decisions using this statistical comparison and does not consider other qualitative factors that may be important to the decision-making process.

⚖️ Statistical Framework and Data Presumptions

This practice derives the decision point and confidence limit in the framework of a statistical test of hypothesis under three different presumptions. The relationship between the decision point and the confidence limit is also described in this context.

A fundamental requirement is that the data are normally distributed. When this assumption does not apply, a transformation to normalize the data (e.g., lognormal) may be needed. The practice explicitly does not cover the determination of decision points and confidence limits for statistics other than the mean concentration. Furthermore, if nonparametric methods are used or when there are many data points below the detection limit, the methods in this practice may not apply.