Precision estimates and suggested sample sizes for length-frequency data
Gerritsen, Hans D. | McGrath, David
For most fisheries applications, the shape of a length-frequency distribution is much more important than its mean length or variance. This makes it difficult to evaluate at which point a sample size is adequate. By estimating the coefficient of variation of the counts in each lengthclass and taking a weighted mean of these, a measure of precision was obtained that takes the precision in all length classes into account. The precision estimates were closely associated with the ratio of the sample size to the number of size classes in each sample. As a rule-of-thumb, aminimum sample size of 10 times the number of length classes in the sample is suggested because the precisiondeteriorates rapidly for smaller sample sizes. In absence of such a rule-of-thumb, samplers have previouslyunder-estimated the required sample size for samples with large fish, while over-sampling small fish of the same species.
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