For determining systematic error in a laboratory's monthly means, which plot is utilized?

The Levy-Jennings plot is specifically designed for monitoring the performance of assays over time, particularly in identifying systematic errors in laboratory results. This type of chart plots the mean of the results alongside the control limits, allowing laboratories to visualize how their monthly means compare to expected values.

Using a Levy-Jennings plot helps in quickly assessing whether any systematic shifts or trends are present in the data, which is essential for maintaining assay accuracy and reliability. It is particularly useful in quality control because it highlights changes in laboratory performance, such as a sudden deviation from the established mean or consistent shifts in measurements.

In contrast, the Tonks-Youden plot is a method for assessing the precision and bias of a method in a more generalized context, focusing on comparisons between different methods or laboratories rather than tracking systematic error over time. The Cusum plot is utilized for monitoring changes in the process mean and detecting shifts, but is often more complex and less straightforward for monthly mean analysis. Linear regression is used for examining relationships between variables, which is not specifically aimed at individual assay performance over time.

Therefore, for systematic error assessment through monthly means, the Levy-Jennings plot is the most appropriate tool.