I am accustomed to seeing estimates of average condition instead of estimates of proportion. Can sample survey data be used to estimate average condition?
Yes, estimates of average condition, such as the average IBI in a watershed, provide valuable information and can be calculated with sample survey data as a simple mean. The principles of survey sampling, particularly the emphasis on selecting a representative sample, also apply to estimating population mean. Just as an estimate of the percent of stream miles in a Region in which IBI is below 40 is biased if data are collected only from sites downstream of sewage outfalls, so is the estimate of mean IBI. Furthermore, estimates of various other properties can be made from the sample survey results, such as median scores, various percentiles, or frequency distributions and their shapes. EMAP emphasizes estimating spatial extent (e.g., percent of river miles) because it has several advantages over estimating the mean alone. For instance, a Region with an average stream IBI of 45 might be composed entirely of streams with an IBI of 45; however, the same average would occur if half the stre
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