J. Laake, Distance sampling with independent observers: Reducing bias from heterogeneity by weakening the conditional independence assumption, MARINE MAMMAL SURVEY AND ASSESSMENT METHODS, 1999, pp. 137-148
Using replicated surveys of a known population of wooden stakes in a sagebr
ush habitat, I illustrate the application of surveys which combine sight-re
sight with line transect sampling. This example demonstrates that the usual
assumption of full conditional independence (FCI) for all distances (x gre
ater than or equal to 0) is unnecessarily strict and can be weakened to ass
uming conditional independence on the trackline (x = 0). I present a likeli
hood for combined sight-resight and distance sampling based on trackline co
nditional independence (TCI) and use a likelihood ratio test for comparison
of the TCI and FCI models. For the wooden stake surveys, in most cases the
TCI model was superior to the FCI model because of heterogeneity in detect
ion probability created by variation in vegetation density for stakes at x
> 0. Equivalent factors in marine mammal surveys, such as sea state and clo
ud cover, will create heterogeneity in detection probability that will bias
abundance estimates developed with combined distance and sight-resight sur
veys under the FCI assumption.