Which two metrics are used to evaluate production water?

• A. Geometric Mean and Maximum Contaminant Level
• B. Arithmetic Mean and Statistical Threshold Value
• C. Geometric Mean and Statistical Threshold Value
• D. Median and Maximum Contaminant Level

Answer: (C)

When evaluating production water, you want to understand both what is typical and how much variability there is across samples. Microbial data from water tend to be skewed, with some higher outliers, so using the geometric mean gives a central tendency that reflects the typical level without being unduly pulled up by rare high results. That makes the geometric mean a better summary of the usual water quality than the arithmetic mean for this kind of data.

At the same time, you need a rule to decide when an individual sample indicates a potential problem. The Statistical Threshold Value serves as a threshold for single samples within a sampling plan; if a sample exceeds this threshold, it triggers a corrective action or investigation. This addresses the risk of spikes that the average alone might miss.

Using both metrics together—geometric mean to describe the typical level and STV to flag excursions—provides a balanced view of production water quality: it shows what is normally observed and when unusual results require attention. The other pairings don’t fit as well because they either rely on an arithmetic mean, which isn’t ideal for skewed microbial data, or use regulatory limits intended for single-sample compliance (like a maximum contaminant level) without addressing the pattern across multiple samples, or rely on a median that’s less commonly used in this context.