1st Order Jackknife 
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Independently, Heltshe & Forrester (1983) and Burnham & Overton (1978) developed the firstorder jackknife estimator:
where n is the number of samples and a the number of species only found in one sample. Heltshe & Forrester (1983) give the variance of this estimate as:
where fj is the number of samples holding j of the L species only found in one sample. Further jackknife estimators are discussed in Burnham & Overton (1979) and Smith & van Belle (1984). Experience suggests that this is a good generalpurpose estimator of species richness.
The output presents a series of estimates using 1, 2, 3... and so on samples. Thus to get sensible estimates you must ensure that all the data come from samples obtained from the same community.
A simple plot of the way the estimate changes with the number of samples used is shown by clicking on the Graph tab.
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.
