Chao Quantitative 
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This is the first of the series of estimators of species richness. The output presents a series of estimates using 1 sample, then 2 samples, then 3 samples etc. Thus to get sensible estimates you must ensure that all the data come from samples obtained from the same community.
If Number of Random Selections (R) is set > 1 then the sequence samples are selected R times at random from the complete set of samples and the mean estimate calculated. As this removes sample order effects it is generally to be recommended.
By looking at the progressive change in the estimates it is possible to assess if sufficient samples have been taken to stabilise the estimate.
A simple plot of the way the estimate changes with the number of samples used is shown by clicking on the Graph tab in the results window.
At present there are no clear indications as to which of these measures is superior and they all should be used with caution. The Chao estimators seem to be some of the best available.
Using the observed number of species represented by one, a, or two, b, individuals in the sample Chao (1984) derived the simple estimator:
Equ(1)
where Sobs is the actual number of species in the sample, a the number of species represented by a single individual, and b the number of species represented by two individuals. Chao (1987) gives the variance of this estimate as:
Equ(2)
Note that when all the species have been observed more than twice, the census is considered complete.
