What can i do to fix my ecological survey sample size with high variance?

[ScottEG]

I’m trying to determine the appropriate sample size for my ecological survey, but my pilot data is showing a much higher variance in species counts than I anticipated. I’m worried my initial power calculation is now way off, and I’m not sure how to adjust my sampling protocol to achieve statistical significance without making the fieldwork completely unmanageable.

[Timothy_R]

Yeah I ran into the same thing last season. Our pilot showed a lot more scatter in counts than we expected, so the power numbers were useless. We tried a stratified design across three habitat types and bumped the number of plots from 10 up to 20. We kept visits per plot modest and still managed field days, and the wider sampling helped stabilize the estimates, at least a bit.

[MiaC]

I re-did the power with the observed spread using a dispersion parameter, and it said you’d need way more plots than we can realistically do if we want to detect a small effect. We ended up deciding to look for a bigger effect or accept a wider confidence interval.

[Brandon_P]

We switched to a generalized linear mixed model with a negative binomial error structure and site as a random effect. It helped account for extra-Poisson variation, but it didn’t magically reduce the data collection effort.

[Richard.G]

I started wondering if observer error is a bigger driver than biology. Two people counting the same patch gave different tallies about 20 percent of the time. We did a quick training run and paired counts, which slowed things but cut the disagreement a bit.

[MilaVP]

We also reframed the goal from reaching a p value to estimating a credible effect with a reasonable interval. We defined a practical minimal detectable difference and used that to guide whether additional sampling was worth it.

[MilaS]

Honestly the field plan started to feel like a logistics puzzle more than a biology puzzle. I kept promising myself we’d get back to the science once the trucks and weather cooperated, then the season rolled on.

[ZacharyQS]

One quick question, is the sampling unit nested in habitat types or are you mixing incompatible units?