The probability of failing to detect a trend when 1 exists has been conside
red only rarely in the interpretation of monitoring studies. Retrospective
power analysis accomplishes this assessment. me apply retrospective power a
nalysis to evaluate both population trends and survey design in the waterfo
wl surveys conducted by the Canadian Wildlife Service and others around Ris
ke Creek, British Columbia. Eleven of 18 species showed long-term (17 yr) a
nd short-term (10 yr) trends. For the remaining 7 species, the long-term an
alysis had sufficient power (0.8) to have detected at least a 5% annual cha
nge, had 1 existed, which supported the conclusion that little change occur
red. However, statistical power and detectable effects varied considerably
among species, with a range of 3-14 years of data needed to be able to dete
ct a 5% annual trend. When we used the shorter-term dataset, power was redu
ced below acceptable levels for 4 of the 7 species failing to show a trend.
It would be a mistake to conclude that the numbers of these 4 species were
not changing. Statistical power was highest for the species for which the
surveys were originally designed, Barrow's goldeneye (Bucephala islandica)
and mallards (Anas platyrhynchos), which had narrow confidence intervals an
d relatively small minimum detectable trends. In contrast, blue-winged teal
(Anas discors), gadwall (Anas strepera), green-winged teal (Anas crecca),
northern pintail (Anas acute), and northern shoveler (Anas clypeata) had re
latively large minimum detectable trends and nide confidence intervals. Muc
h of the power of these surveys was due to repeated surveying within season
s. For most species, power increased substantially by including up to 4 sur
veys as replicate observations within a year, but power increased little wh
en data from a fifth or sixth survey were included.