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This test is suitable for open populations that may have experienced both mortality and recruitment. However, it is only based on animals that were captured on the first sampling occasion that were also captured on the last sampling occasion. It therefore only uses part of the available data, and if few animals meet this criterion it cannot be applied. This test is automatically calculated whenever an open population estimation method is selected and full individual tagging data are available. The results can be viewed by clicking on the Equal Catchability Open tab. If fewer than 20 animals are available for analysis, the calculations are still undertaken, but a warning is given that the test is unreliable. A comparison is made of the actual and expected variances of a series of recaptures of individuals known to be alive throughout the sampling period; individual marks must have been used.
This test is best illustrated by a worked example taken from Leslie's appendix, based on the recaptures of shearwaters; with insects, of course, the recapture periods would be days or weeks rather than years.
32 individuals marked in 1946 were recovered for the last time in 1952, therefore they were available for recapture in the years 1947-51 inclusive, and the following two tables were prepared:
Year by year analysis
Year No. of recaptures in
each year (ni)
All years analysis
No. of recaptures
for each individual X Frequency of X f(x)
N = åf(x) = 32
The actual sum of squares:
= 69-30.03 = 38.97.
The expected variance :
= 31/32-199/322 = 0.9688-0.1943 = 0.7745
Then X2 = 38.97/0.7745 = 50.32.
X2 may be treated as equivalent to Chi-squared, and for degrees of freedom (N - 1) of between 20 and 30 the probability of a value as great or greater than this can be assessed from C2 tables. Leslie has suggested that the test should only be used when the number of individuals is 20 or more and the number of occasions on which recapture was possible is at least 3. This test will not distinguish whether the higher catchability of some individuals is due to catching effects or inherent individual differences.