What is a sample that has been built up from a series of samples taken at intervals during a fixed sampling period called?

A sample that has been built up from a series of samples taken at intervals during a fixed sampling period is referred to as a composite sample. This method involves combining multiple individual samples collected over time into one single sample for analysis. The purpose of creating a composite sample is to obtain a more representative measurement of the entire time period being studied, as it smooths out fluctuations that may occur from varying conditions such as flow rate or pollutant concentration.

For instance, in wastewater analysis, if you were to collect samples every hour for a day and then merge them into one single sample, this would yield a composite sample. This approach helps in understanding average concentrations of pollutants over time rather than relying on a snapshot from a single moment, which may not accurately reflect the typical water quality during the sampling period.

Other terms like "single sample" or "incremental sample" do not adequately describe this process. A single sample refers to a one-time collection without consideration for time variations, while incremental sampling generally refers to a method of collecting samples at various depths or locations rather than over time. Thus, the term "composite sample" most accurately captures the essence of what was described in the question.