A grab sample is a sample collected at a specific time and place. Grab samples collected at suitable intervals may be used to document variations in time and space. A composite sample is a mixture of samples collected at different times from the same sampling point. Composite samples are useful for observing average concentrations. For instance if the discharge permit for a facility specifies an average value over a 24 hour period, composite sampling might be appropriate.
Composite samples save analyses time and money.
Integrated samples are collected from different points simultaneously. The answers to these questions will help us decide the number of samples, duplicates, and blanks which will be required. Duplicates and blanks are discussed later. For example if we were trying to estimate excess cancers in a neighborhood, the total population of the neighborhood would be the sampling domain and the sampling unit would be each individual. For sampling geographical areas it is generally easiest to superimpose a grid on a map of the area.