Understanding the results

questions concerning analysis/theory using program PRESENCE

Understanding the results

Postby Sarah_Walker » Sat Aug 20, 2022 12:27 pm


I am currently trying to interpret my results but am having difficulty understanding how some of the results are obtained. Primarily I need to extract the results which show how many times a site would need to be visited to allow detection of the species, as well as how many times it would need to be visited to confirm that the species in absent from the site.

If this information is too vague please let me know. I am in the last few weeks of my MSc Primate Conservation thesis and after a later field season than planned, so any help would be greatly appreciated.

Thank you in advance for any help
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Re: Understanding the results

Postby jhines » Sun Aug 21, 2022 11:07 am

To determine how many times you need to visit a site to allow detection of a species, you need to compute "p*", which is the probability that the species is detected at least once, given the species occupies the site. The formula for p* is:

p* = 1 - (1-p1)(1-p2)(1-p3)...(1-pk)

where p1-pk are the survey-specific detection probabilities, and k is the number of surveys. If all detection probabilities are the same then,

p* = 1 - (1-p)^k

Unless p = 1, p* will be less than 1, so it would require an infinite number of surveys to confirm that a species present or absent. Otherwise, you have to decide how close to 1.0 is acceptable for you. For example, if p is constant at 0.5, and k=4, then p* = .9375. If k=5, the p*=0.96875 and you would be >95% certain that you would detect the species if it was there with 5 surveys.

The 95% level in that example might seem extreme. The way the question is worded, you might want to know how many surveys are needed to get a reasonable chance of detecting the species. So, you might be satisfied with 50%. If the species is difficult to detect, you might even be satisfied with 10 or 20%. The lower the probability of detection, the more sites will be needed to get accurate estimates of occupancy.
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