FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

e. Conclusion. The preliminary sample of 12 observations was
adequate. If the preliminary sample of 12 observations had been
inadequate, the manager had several options. First, more observations
could have been taken. Second, the acceptable precision level could
have been lowered. Third, a lower confidence level could have been
accepted and some added risk incurred. Or fourth, the fire prescription
might have been altered (e.g., faster windspeed or lower fuel moisture
content) to compensate for the added uncertainty. The important point is
that the manager had some quantitative information on which to base a
decision.
f. Note. The size of the preliminary sample depends on the variable
being measured, on the objective for measuring the variable, and on
funds, time, and work force constraints. Inherent variability associated
with most natural resource sampling indicates that 10 or more
observations may usually be required to obtain a reasonable estimate of
variance. One observation is never adequate because degrees of
freedom would be zero and no analysis is possible.
2. t-Test for Paired Observations (Plots). This test is especially useful
for comparing effects between two fire treatments, such as burned vs.
unburned, or backing fires vs. heading fires. An assumption of the test is
that pairs of plots are established prior to the treatment, each group has
the same population variance, the population of observations follows the
normal distribution, and that treatments are randomly assigned to each
individual plot. It is possible, however, to establish plot pairs adjacent to
firelines (one plot on either side) such that these assumptions are not
violated. A similar test for unpaired plots (Freese 1967, p. 24) should be
used where plots are not paired; that test, however, is slightly more time
consuming and a slightly greater loss of sensitivity may occur. Only the t-
test for paired plots is illustrated here.
Example: A fire manager suspected that fire residence time might be an
important factor affecting postburn Idaho fescue (Festuca idahoensis)
production. The manager designed an experiment to evaluate the
suspicion. An area was selected where part of the burning would be
done by a backing fire and part by a heading fire. A series of 10 plots
were clipped, oven-dried, and weighed preburn to establish that the two
areas were from the same population and had similar variances, and to
determine preburn fuel loading. The heading and backing fires were
then conducted on the same day under similar environmental conditions.