Sample size

[Rodrigo Moncayo Estrada]

I applied the NPMR with historical data collected in 1979 in one lake and 1995 in another lake for the fish community and environmental variables. I am particularly interested in one threatened species. The samples size were: 5x4=20 and 4x4=16, respectively (sites and the number of times the samples were collected in each site). I am concern about how the small sample size and the non-independence of data (they have some aggregation) could affect the performance of the NPMR. Perhaps one direct recommendation is to increase the sample size, but this is the only complete available data of the species in the first lake, before it become locally extinct; and because of the restricted condition in the second lake it is difficult to get more data. Any feedback would be greatly appreciated. Thank you.

[Bruce McCune]

Rodrigo, I'm assuming that you wish to evaluate how the performance of the rare species varied in relationship to the environmental measurements and date. If you had sampled both sites at both dates you could represent "site" with a random effect -- including a variable coding for site in the model. But because site is confounded with date, about all you can do is just describe the variation in the threatened species in relationship to the environmental variables, then explain to the reader/listener that the analysis is exploratory and that you can't separate the change through time from differences between lakes. 

Others might have ideas on this too -- I think it's a case where it would be nice to use the data and the results might be suggestive but cannot be conclusive.

I hope this helps,

Bruce McCune

On Thu, Dec 27, 2018 at 7:39 AM Rodrigo Moncayo Estrada <rodrigomon...@gmail.com> wrote:

I applied the NPMR with historical data collected in 1979 in one lake and 1995 in another lake for the fish community and environmental variables. I am particularly interested in one threatened species. The samples size were: 5x4=20 and 4x4=16, respectively (sites and the number of times the samples were collected in each site). I am concern about how the small sample size and the non-independence of data (they have some aggregation) could affect the performance of the NPMR. Perhaps one direct recommendation is to increase the sample size, but this is the only complete available data of the species in the first lake, before it become locally extinct; and because of the restricted condition in the second lake it is difficult to get more data. Any feedback would be greatly appreciated. Thank you.

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[Rodrigo Moncayo Estrada]

Unfortunately, I have data in both sites but only one date each. Lake Cuitzeo was only complete sampled in 1979 (five sites sampled at four different moments during that year), then the species was not captured anymore. And for Lake Zacapu one sampling event (four sites in four different moments during year 1995) this is the place where the species is now restricted. This is why I’m concern about the sample size. 

[Rodrigo Moncayo Estrada]

Thank you Bruce for your answer it was really helpful

El jueves, 27 de diciembre de 2018, 8:39:18 (UTC-7), Rodrigo Moncayo Estrada escribió: