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The
Probit
Model
of
Modal
Choice
In
the
bimodal
case,
the
probit
model
equates
the
probability
that a
given
traveler
chooses mode 1
instead
of mode
2
to
the
probability
that a
standardized
normal
random
variable
takes
on a
value
less
than
some
critical
level
called
the
"utility"
of mode
1.
The
utility
of
mode 1
is
evaluated
as a
function
of
the
characteristics
of
the
traveler
and
the
relative
advantages
(or
disadvantages)
of mode
lover
mode
2.
Recall
from
your
introductory
statistics
class
that
for a
particular
observation
(or
variate) X*
on a
normally
distributed
random
variable X
with
mean
~
and
variance
oei.e., X 
N(~ra)
the
"z
score"
for
this
observation
is
computed
according
to
the
formula
Z*=(X*~)/o.
Z
scores,
which
are
measured
in
standard
deviation
units
from
the
mean
of X,
or
~,
are
distributed
according
to a
standardized
normal
distributioni.e., Z 
S.N.(Orl). Z
scores
are
tabulated
to
correspond
to a
cumulative
standardized
normal
distribution
which
gives
the
proportion
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This note was uploaded on 03/16/2010 for the course CRP 3210 at Cornell University (Engineering School).
 '09
 DONAGHY,K.

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