Accuracy of Surface Samples from Gravel Bed Material

Operator and sampling errors in the grid by the number sampling technique for river gravels result in differences between sample and population parameters. Differences between operators occur because of slight differences in stone selection procedure and are independent of sample size. Differences between samples occur because of random errors and these decrease with increasing sample size. Consequently, as sample size increases differences between operators become statistically significant even though physically they remain the same. This is illustrated by data for eight operators. Samples of 30, 60 and 100 show no significant differences but those for 120, 180 and 300 do show significant differences. At small sample sizes, sampling error is large and the sample parameters only approximately define population values. The sample size required to produce a prescribed level of accuracy can be determined from statistical theory and the equations needed are presented here. Where samples larger than 100 stones are used it will be necessary to minimize differences between operators by thorough training. For very large samples and high accuracy one operator must collect all the stones.