The variability of estimates of variance, and its effect on power analysis in monitoring design.

Power analysis can be a valuable aid in the design of monitoring programs. It requires an estimate of variance, which may come from a pilot study or an existing study in a similar habitat. For marine benthic infauna, natural variation in abundances can be considerable, raising the question of reliability of variance estimates. We used two existing monitoring programs to generate multiple estimates of variance. These estimates were found to differ from nominated best estimates by 50% or more in 43% of cases, in turn leading to under or over-estimation of sample size in the design of a notional monitoring program. The two studies, from the same general area, using the same sampling methods and spanning a similar time scale, gave estimates varying by more than an order of magnitude for 25% of taxa. We suggest that pilot studies for ecological monitoring programs of marine infauna should include at least two sampling times.