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

term contains one bracketed expression, (8b 1-1), which is negative
by assumption. If this expression dominates, the overall intercept
term will be negative. It should also be noted that whenever the in-
tercept is estimated as negative the dummy variable coefficient is es-
timated to be positive.
In general we expected the mileage coefficients to be negative.
Our theory assumes that for each commodity group the total amount
transported would decrease with distance. This assumption was valid.
However, if one mode enjoys a great advantage in the shorter hauls and
another in the longer ones, the latter mode may display a positive dis-
tance coefficient and remain consistent with our theory so long as
this positive effect is more than compensated for by the reduction in
tonnage for the former mode. Two of the nine estimated coefficients
came out positive and both of these were offset by negative intercept
terms. Water shipments of Group 29, Petroleum and Coal Products, in-
creased rather largely with distance and rail shipments of Group 30,
Rubber and Miscellaneous Plastics Products, increased a very small
amount. However, all of the statistically significant estimates showed
a negative mileage effect.
Without exception the signs of the estimated coefficients of the .
square of distance were the opposite of the sign of the distance co-
efficients. This coefficient, however, was never large enough in value
to completely override the influence of the first degree term. That
is, if the distance coefficient was negative and the distance squared
coefficient positive, for the relevant range, the net effect of dis-
tance on quantity was negative.
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