Representative Samples

P.T. Weiss, A.J. Erickson, J.S. Gulliver, R.M. Hozalski, O. Mohseni, and W.R. Herb

Regardless of the type of samples collected (in situ, on-site, grab, or automatic), it is imperative that representative samples are measured or collected. A representative sample is a sample in which the measured parameter (e.g., phosphorus) is the same in the sample as in source from which the sample was measured or collected. In many cases, samples are only representative for a very short period of time and a small, specific location.

To make conclusions in an assessment program, it may be necessary to make assumptions about the dynamics of a system and to what degree a collected sample is representative. When planning a sampling program to include representative samples, consider the following:

  • How will sample contamination be prevented?
  • Does the measured parameter (e.g., phosphorus) change drastically in time or space?
  • Do conditions other than the measured parameter (e.g., discharge) change drastically in time or space?
  • Is the system poorly mixed or very large (such that a sample in one location is not representative of the entire system)?

In order to measure or collect representative samples, it may be necessary to use a specific (or several) sampling method. Several examples for choosing sampling methods to ensure representative samples are given below:

  • Measuring dissolved oxygen with on-site, grab, or automatic sampling in some situations requires careful sample collection, storage, handling, and analysis to prevent contamination. It may be more cost-effective to measure dissolved oxygen in situ.
  • Some systems change quickly and capturing representative samples in these systems may require measuring or collecting several samples in a short period of time. On-site and grab sampling may be limited by the capacity of the personnel measuring or collecting the samples and therefore in situ or automatic sampling may ensure representative samples.
  • Some systems vary or change drastically in space and capturing representative samples in these systems may require measuring or collecting samples in several locations. It may be cost-prohibitive to install in situ or automatic samplers in several locations and therefore it may be more cost-effective to measure or collect on-site or grab samples to ensure representative samples.
  • Some systems vary or change drastically in time and space. Capturing representative samples in these systems may require measuring or collecting samples repeatedly in several locations simultaneously. On-site and grab sampling may be limited by the capacity of the personnel measuring or collecting the samples and therefore in situ or automatic sampling may ensure representative samples. It may, however, be cost-prohibitive to install in situ or automatic samplers in several locations. In these situations, the best solution may be to choose a different (often simpler) assessment method or study site.

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