As the newsletter theme attests, there is strong current interest in monitoring the effects of management on watershed resources, providing an opportunity to review a basic principle of data collection. I hope to convince you that sampling directly affects the quality and reliability of the information collected. Worse, it could be misleading. Since sampling applies to any situation where part of a population is measured, it applies to monitoring. Little attention is usually given to sampling, but the results always depend on how it is done.
Estimates calculated from sample data vary since population units comprising the sample vary.
A sampling scheme is evaluated and different schemes are compared by their distributions. The quality of a single estimate can be judged only by the properties of its distribution of possible outcomes . One way to study sampling schemes is to simulate many samples from a known population using different schemes and compare the resulting distributions. The scales are identical and the vertical line through all plots gives the known true load of 3058 metric tonnes. Statistics for each scheme are given in the table.