Sampling from a larger sample

[Maureen]

I have samples of aquatic invertebrates that need to be measured so that I can calculate length-weight conversions and estimate mass.  The samples have been collected to calculate the mass of invertebrates in a given volume of water at different times of year.  Some samples have just a few invertebrates -- no problem, measure all of them.  Other samples have 100s to 1000s of invertebrates, and I can't afford to measure them all.  So I have a two-part question:

1. Which is more accurate/makes more sense: a) Randomly choosing a certain percentage of invertebrates from the large samples (e.g., choosing 10% from each sample), or b) Randomly choosing a set number of invertebrates to measure (e.g., randomly choosing 20 invertebrates, no matter the sample size for all samples greater than 20)?

2. How do I figure out what percentage (if the answer to the above question is a) or what number (if the answer to the above question is b) will adequately represent the population?

I would appreciate any help to get me pointed in the right direction!

[Lenth, Russell V]

I am on a trip and can't answer in detail. But as I understand it you are trying to sample from a sample. So the right answers depend in part on how the original sample was collected. Also important are whether there are any covariates involved, why you have such big discrepancies involved, and in general what your goals are. Keep in mind that your goal is probably still to make an inference about the underlying population from which the original sample was drawn. I hope somebody can help you further. 

Russ

Sent from my iPhone

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