Q2 Z2,(Q2) Z2(Q2) - Institute for Water Resources - U.S. Army

Since the aim of this study is to examine how the selected variables,
individually and jointly, help to determine or to explain the level of
rail rates, we have entered these variables into our regression equa-
tions in a stepwise fashion. This allows us to observe, and test, the
increase in explanatory power of the equation as each additional variable
is entered, and also to observe the stability or robustness of the co-
efficients of previously entered variables. The sequence in which the
variables enter the equation was determined beforehand and no variable,
once having entered the equation, was allowed to drop out. The enter-
ing sequence was, in almost all cases, the same as would have occurred
had we chosen the variable with the largest "F to enter" statistic at
each step.
In earlier studies, we had found linear equations usually provided
a better fit than semi-log or log-linear equations. For this case our
regression equations were
(4.33) R ■ S+SMe and
ij 10 11 ij lij'
(4.34) R ij
where
Rij
B + 0 M + 0 Q + e
• 20 21 ij 22 ij 2ij
• average rate in cents-per-ton, for shipments
• from state i to state j;
M ij ■ average distance, in miles, traveled by
shipments from state i to state j; .
Q ij • annual tonnage in thousands of tons, shipped
from state i to state j;
0 the n th coefficient to be estimated in the m th
mn
equation; and
■ the disturbance terms, (k ■ 1, 2).
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