// File: 02weight.mod
// Author: Michel Bierlaire, EPFL
// Date: Fri Nov 12 16:44:22 2010
// Logit model
// Three alternatives: Train, Car and Swissmetro
// SP data
[ModelDescription]
"Example of a logit model for a transportation mode choice with 3 alternatives:"
"- Train"
"- Car"
"- Swissmetro, an hypothetical high-speed train"
"We illustrate a case where persons from GROUP 3 have been undersampled."
"In order to apply the WESML estimator, weights must be associated to observation."
"A weight of 1.0 is associated with GROUP 2 and a weight of 1.2 with GROUP 3"
"Note that the valie of the log likelihood cannot be compared with the ESML value."
[Choice]
CHOICE
[Weight]
// To illustrate the syntax for weight, we assign a weigh of
// 1.2 to all observations corresponding to group 3. The weight is also
// normalized so that the sum across the sample is the sample size.
8.890991e-01 * (1.0 * (GROUP == 2) + 1.2 * (GROUP == 3))
[Beta]
// Name Value LowerBound UpperBound status (0=variable, 1=fixed)
ASC_CAR 0 -10 10 0
ASC_TRAIN 0 -10 10 0
ASC_SM 0 -10 10 1
B_TIME 0 -10 10 0
B_COST 0 -10 10 0
[LaTeX]
ASC_CAR "Cte. car"
ASC_SBB "Cte. train"
ASC_SM "Cte. Swissmetro"
B_TIME "$\beta_\text{time}$"
B_COST "$\beta_\text{cost}$"
[Utilities]
// Id Name Avail linear-in-parameter expression (beta1*x1 + beta2*x2 + ... )
1 A1_TRAIN TRAIN_AV_SP ASC_TRAIN * one
+ B_TIME * TRAIN_TT_SCALED
+ B_COST * TRAIN_COST_SCALED
2 A2_SM SM_AV ASC_SM * one
+ B_TIME * SM_TT_SCALED
+ B_COST * SM_COST_SCALED
3 A3_Car CAR_AV_SP ASC_CAR * one
+ B_TIME * CAR_TT_SCALED
+ B_COST * CAR_CO_SCALED
[Expressions]
// Define here arithmetic expressions for name that are not directly
// available from the data
one = 1
CAR_AV_SP = CAR_AV * ( SP != 0 )
TRAIN_AV_SP = TRAIN_AV * ( SP != 0 )
//If the person has a GA (season ticket) her incremental cost is actually 0
//rather than the cost value gathered from the
// network data.
SM_COST = SM_CO * ( GA == 0 )
TRAIN_COST = TRAIN_CO * ( GA == 0 )
// For numerical reasons, it is good practice to scale the data to
// that the values of the parameters are around 1.0.
// A previous estimation with the unscaled data has generated
// parameters around -0.01 for both cost and time. Therefore, time and
// cost are multipled my 0.01.
TRAIN_TT_SCALED = TRAIN_TT / 100.0
TRAIN_COST_SCALED = TRAIN_COST / 100
SM_TT_SCALED = SM_TT / 100.0
SM_COST_SCALED = SM_COST / 100
CAR_TT_SCALED = CAR_TT / 100
CAR_CO_SCALED = CAR_CO / 100
[Exclude]
// All observations verifying the following expression will not be
// considered for estimation
// The modeler here has developed the model only for work trips.
// Observations such that the dependent variable CHOICE is 0 are also removed.
(( PURPOSE != 1 ) * ( PURPOSE != 3 ) + ( CHOICE == 0 ))
[Model]
// $MNL stands for "multinomial logit model",
$MNL