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

APPENDIX A TO CHAPTER FOUR
The Effects of Annual Tonnages Upon Rail Rates
The behavior of the annual tonnage coefficient estimates in the
rail regressions requires special consideration. In the first two
sections of that analysis (those dealing with all commodities and with
commodity group totals) the tonnage coefficients were always estimated
with a negative sign and all but four were statistically different from
zero. The estimates, did not behave so well in the final set of re- '
gressions. With the less aggregate data, and including variables rep-
resenting price and density, six of the ten tonnage coefficients were
estimated as positive. Only two of these were significantly positive
but none of the four negative coefficients was significant.
Upon observing these estimates it was decided to investigate the
effect of the quantity variable further. A new set of regressions was
run for each commodity group using the sane data but forcing the tonnage
variable to enter the estimating equation at the second step, distance
• -.being retained as the first independent variable. Tables 4.A.1 through
4.A.5 contain the results of these runs.
The tonnage coefficient estimates definitely are not stable. They
shift greatly in size and frequently change sign. For Groups I and V,
the estimates are at first negative and then, as other variables enter
the equation, become positive. For Groups III and IV, the estimates
are first significantly negative and then move in the direction of
positiveness, losing their significance in the process. The tonnage
coefficient for Group II first appears positive, then goes negative,
and finally returns positive, never achieving significance.
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