| Popis: |
One motivation for estimating the standard error, SEMw, of a weighted mean concentration, Mw, of an ion in precipitation is to use it to compute a confidence interval for Mw. Typically this is done by multiplying the standard error by a factor that depends on the degree of confidence one wishes to express, on the assumption that the weighted mean has a normal distribution. This paper compares confidence intervals of Mw concentrations of ions in precipitation, as computed using the assumption of a normal distribution, with those estimated from distributions produced by bootstrapping. The hypothesis that Mw was normally distributed was rejected about half the time (at the 5% significance level) in tests involving nine major ions measured at ten diverse sites in the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Most of these rejections occurred at sites with fewer than 100 samples, in agreement with previous results. Nevertheless, the hypothesis was often rejected at sites with more than 100 samples as well. The maximum error (relative to Mw) in the 95% confidence limits made by assuming a normal distribution of the Mw at the ten sites examined was about 27%. Most such errors were less than 10%, and errors were smaller at sampling sites with > 100 samples than at those with < 100 samples. |
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