What type of sample takes individual discrete samples and mixes them together to create a well-mixed average concentration sample for an entire sample area?

• A. Grab sample
• B. Composite
• C. Sieve
• D. Pooled sample

Answer: (B)

Composite sampling is used to obtain a representative average concentration across an area by taking multiple discrete samples from different locations within that area and mixing them together. This approach averages out local variations, giving a single sample that reflects the overall conditions rather than a single spot’s values. By thoroughly combining the collected subsamples, you get a well-mixed sample that represents the entire area being studied.

This differs from a grab sample, which is taken from one location at one time and reflects only that spot’s conditions. The sieve is a device for separating particle sizes, not a sampling strategy. A pooled sample can refer to combining subsamples to increase volume but doesn’t inherently ensure a true average across the area unless the subsamples are collected and mixed in a way intended to represent the whole area, which is what composite sampling explicitly does.