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

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

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CHAPTER II<br />

Discrimination With One Variable<br />

(1)<br />

Since the particular problem relevant tO later statistical<br />

work involves a choice among three modes, most of this chapter will<br />

deal with this particular case. However, it should be noted that<br />

the theoretical equations could be readily applied to a general<br />

situation <strong>for</strong> n modes, even though examples are confined to n a 3•<br />

Let us define w 1 as the population of origins which ship<br />

by barge, 11 2 as the population of origins which ship by rail, and<br />

11 3 as the population of truck transportation origins. Each origin<br />

is characterized by three variables: X 21 , the difference between<br />

rail transportation net price and barge transportation net price,<br />

X31 the difference between truck and barge net prices, and X32 ,<br />

the similar difference <strong>for</strong> truck and rail. p i (X), p2 (X), p 3 (X)<br />

are the multivariate density functions of the respective populations,<br />

where X is the vector of origin characteristics (X21 , X31 , X32 ).<br />

Next, qi , q2 , q3 are the a priori probabilities of drawing an<br />

observation (any observation) from the respective populations: they .<br />

correspond to the relative frequencies of each population in the<br />

universe of the three populations. For the time being, it is assumed<br />

that both the p i(X)'s and the qi(X)'s are known.<br />

Given an observation X, the conditional probability that<br />

X comes, say, from w i (the probability that an origin with characteristic<br />

vector X ships by barge) is<br />

■ P.m%<br />

ql p1(X)<br />

1 3<br />

qi p i (X)<br />

i=1<br />

-14-

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