Descriptive Statistical Techniques

The air quality in a city in terms of, say, the level of sulphur dioxide present, cannot be adequately assessed by a single measurement. This is because air pollutant concentrations in the city do not have a fixed value but vary from one place to another. They also vary with respect to time. Similar considerations apply in the assessment of water quality in a river in terms of, say, the level of nitrogen or number of faecal coliforms present, or in assessing the activity of a radioactive pollutant. In such situations, while it may be that some of the variation can be attributed to known causes, there still remains a residual component which cannot be fully explained or controlled and must be regarded as a matter of chance. It is this random variation that explains why, for instance, two samples of water, of equal volume, taken at the same point on the river at the same time give different coliform counts, and why, in the case of a radioactive source, the number of disintegrations in, say, a 1-min time interval varies from one interval to another.