# 114: Swissmetro Nested with Sampling Bias¶

This example is a mode choice model built using the Swissmetro example dataset. First we create the DB and Model objects. When we create the DB object, we will redefine the weight value:

d = larch.DB.Example('SWISSMETRO')
m = larch.Model(d)


We can attach a title to the model. The title does not affect the calculations as all; it is merely used in various output report styles.

m.title = "swissmetro example 14 (nested logit with sampling bias)"


The swissmetro dataset, as with all Biogeme data, is only in co format.

from larch.roles import P,X
m.utility[1] = ( P.ASC_TRAIN
+ P.B_TIME * X.TRAIN_TT
+ P.B_COST * X("TRAIN_CO*(GA==0)") )
m.utility[2] = ( P.B_TIME * X.SM_TT
+ P.B_COST * X("SM_CO*(GA==0)") )
m.utility[3] = ( P.ASC_CAR
+ P.B_TIME * X.CAR_TT
+ P.B_COST * X("CAR_CO") )


To create a new nest, we can use the new_nest command, although we’ll need to know what the alternative codes are for the alternatives in our dataset. To find out, we can do:

>>> m.df.alternatives()
[(1, 'Train'), (2, 'SM'), (3, 'Car')]


For this example, we want to nest together the Train and Car modes into a “existing” modes nest. It looks like those are modes 1 and 3, so we can use the new_nest command like this:

m.new_nest("existing", parent=m.root_id, children=[1,3])


We’re also going to insert a sampling bias correction in this model. To read more on this topic, please see Bierlaire, Bolduc, and McFadden (2008) The estimation of generalized extreme value models from choice-based samples, in Transportation Research Part B: Methodological.

m.samplingbias[1] = P.SB_TRAIN


Larch will find all the parameters in the model, but we’d like to output them in a particular order, so we want to reorder the parameters. We can use the reorder method to fix this:

m.reorder_parameters("ASC", "B_", "existing",)


We can estimate the models and check the results match up with those given by Biogeme:

>>> result = m.maximize_loglike()
>>> print(result.message)
Optimization terminated successfully...

>>> print(m.report('txt', sigfigs=3))
=========================================================================================...
swissmetro example 14 (nested logit with sampling bias)
=========================================================================================...
Model Parameter Estimates
-----------------------------------------------------------------------------------------...
Parameter       InitValue       FinalValue      StdError        t-Stat          NullValue
ASC_TRAIN        0.0            -10.2            3.13           -3.27            0.0
ASC_CAR          0.0            -0.317           0.0443         -7.16            0.0
B_TIME           0.0            -0.0113          0.000576       -19.6            0.0
B_COST           0.0            -0.0111          0.000511       -21.6            0.0
SB_TRAIN         0.0             10.4            3.15            3.31            0.0
existing         1.0             0.874           0.035          -3.61            1.0
=========================================================================================...
Model Estimation Statistics
-----------------------------------------------------------------------------------------...
Log Likelihood at Convergence           -5169.64
Log Likelihood at Null Parameters       -6964.66
-----------------------------------------------------------------------------------------...
Rho Squared w.r.t. Null Parameters      0.258
=========================================================================================...


Tip

m = larch.Model.Example(114)