| Summary: | In British Columbia, government, industry and consulting firms have used taper functions
since the late sixties. Most recently, Kozak's (1988) variable exponent model has been used since
1989. One practical problem with the model is that, it does not estimate total or merchantable
volume without bias. These biases were found to be more pronounced for red cedar (Thuja
plicata Donn ex D.Don) and western hemlock (Tsuga heterophylla (raf). Sarg.). Because of this
problem, a second equation known as the 1994 equation was developed. However, reviewers
identified some theoretical problems concerning multicollinearity and autocorrelation in the 1994
equation. These prompted the development of a third equation that possesses a lesser amount of
multicollinearity referred to as the 1995 equation.
The three principal objectives of this research were: (1) to study the effects of
multicollinearity and autocorrelation on the predictive ability of the 1994 and 1995 variableexponent
taper functions; (2) to study the effects of four sampling strategies on the predictive
ability of the 1994 and 1995 taper equations; and (3) to examine the possibility of localizing the
1994 taper equations.
The effects of multicollinearity and autocorrelation and the four sampling designs were
studied using Monte Carlo simulations. The results of the study indicated that the presence of
severe multicollinearity and autocorrelation in the data did not seriously affect the predictive
ability of the equations.
Stratified random sampling, with equal allocation of observations selected from each
stratum, gave the smallest variability of the estimated coefficients compared to simple random
sampling, and stratified random sampling, with the number of samples proportional to the size of
the strata. However, the average estimated regression coefficients were somewhat different from the population parameters.Therefore, simple random sampling is recommended for selecting
trees from the population if the main objective is the estimation of the population parameters. If
the equations are to be used for prediction, then a wider range of the data (stratified sampling)
should be used.
The results indicated that no adjustment or scaling is required for the western hemlock
equation for the two subzones studied.
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