A better estimate of the standard deviation may often be obtained by the pooling of results from more
than one set. Thus, s may be calculated from K sets of data.
where M = N 1 + N 2 +... NK. One degree of freedom is lost with each set pooled. A common
requirement is the computation of the pooled value for two sets of data only. In this case the simplified
equation (2.4) may conveniently be used:
Standard deviations for results obtained by the arithmetic combination of data will be related to the
individual standard deviations of the data being
Table 2.1 Standard deviations from arithmetically combined datacombined. The exact relation will be determined by the nature of the arithmetic operation (Table 2.1).
The Relative Standard Deviation
Also known as the coefficient of variation , this is often used in comparing precisions.
Variance
The square of the standard deviation (σ^2 or s^2 ). This is often of practical use as the values are additive,
e.g.