MHB Can t-tests be used to compare ecological indices from different sampling sizes?

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The discussion centers on whether t-tests can be used to compare ecological indices derived from different sampling sizes in coastal dune vegetation. The data includes plant species occurrences from 30 sites with varying transect lengths, leading to 30 distinct ecological index values. The user seeks to compare two groups of beaches based on erosive conditions using these indices. Clarification is provided that the indices are species co-occurrence indices calculated from presence/absence data in 0.5 m segments. The main question remains whether the data should be standardized to a common transect length before applying the t-test for comparison.
macedo
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Hello everyone

I would like that you may help me with this following question:

I have data from plant species occurrences from 30 sites (10 beaches with 3 replications for each one; 6 beaches not in erosive process; 4 in erosive process) in coastal dunes vegetation.
At each site I have made data collection along one transect (so all the 30 transects have different lengths and sometimes very different (namely between beaches), the smaller with 8 m and the bigger with 130 m).
I have calculated different ecological indices which gave me naturally 30 different values for each site.
Now, I grouped the 10 beaches in two groups (2 different environmental conditions: erosive beaches (12) and not erosive beaches (18)).
May I compare the two groups with all the data (with all the complete transects) with a t-Test or I have to previously to cut to a common length to all the 30 transects in order to compare statistically the data?
Thanks you for your attention
Sincerely
José A Macedo
 
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macedo said:
Hello everyone

I would like that you may help me with this following question:

I have data from plant species occurrences from 30 sites (10 beaches with 3 replications for each one; 6 beaches not in erosive process; 4 in erosive process) in coastal dunes vegetation.
At each site I have made data collection along one transect (so all the 30 transects have different lengths and sometimes very different (namely between beaches), the smaller with 8 m and the bigger with 130 m).
I have calculated different ecological indices which gave me naturally 30 different values for each site.
Now, I grouped the 10 beaches in two groups (2 different environmental conditions: erosive beaches (12) and not erosive beaches (18)).
May I compare the two groups with all the data (with all the complete transects) with a t-Test or I have to previously to cut to a common length to all the 30 transects in order to compare statistically the data?
Thanks you for your attention
Sincerely
José A Macedo

I think we need to know what the nature of the indices are.

CB
 
hello
Thanks for the message. The indices are species co-occurrence indices (null models) calculated with presence/absence of species in each 0.5 m.
Thank you again
Jose
 

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