The name is absent

17

Due to the truncation, the expectation of %₃ is not zero and must be accounted for in
estimation (Heckman; Greene). The inverse mills ratio

E{%3∣%3 > 0} œ 9(W3)∕F(-W3)

•                # ! #       ∖ #o+!#⁴'^p4     ZOΛT01     __* 1 J * __                  ______

with w₃ ´ — [("₃ — ^.)e + "₃M⁰ ], was included in an additional regressor in

estimating the demand systems (5) to assure that the estimation error has expectation
zero.^&

Results

The estimation results are presented in Table 3. The estimates for all three sites satisfy
the integrability conditions for the parameters to represent a valid demand model, and the
price, quality, and budget parameters are highly significant for the Point Reyes and
Monterey trips, though not so for Half Moon. The model predicts the actual mean trips at
each site relatively well: the predicted (actual) trips for Point Reyes was 2.16 (2.25), for
Half Moon it was 1.15 (1.43), and for Monterey it was 1.64 (1.78). The Half Moon
results are not too surprising in light of the relatively small number of people intercepted
there (72) relative to the other sites, and the fact that there is less variation in the number
of trips taken there.^' The Pt. Reyes and Monterey results, though, illustrate some of the
interesting features of the DS model.

First, the budget parameters "4 are the only ones whose sign directly indicates the
direction of impact of the corresponding demand slope. The significant coefficients (""
and "$) indicate that demand at Point Reyes has a positive income effect, while at
Monterey it has a negative income effect. The finding of negative income effects is

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