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Abstract An expression is derived for the equivalent number of independent values obtained by partial sampling of a coherent, i.e. autocorrelated, time‐series or spatial series of data. The expression is a function of both the coherence of the data values and the coherence of the sampling, i.e. the non‐randomness of the distribution of available data. For a data series with high autocorrelation the distribution of the sampling in time or space is more important than for a relatively incoherent data series. Meteorological data are used as illustrative examples. The derived expression can be used in estimating the variance of sample means.
D. E. Parker (Sun,) studied this question.