# 307: Itinerary Choice with Sampling BiasΒΆ

We begin this example by picking up where we left off on Example 303:

m = larch.Model.Example(303)


That model include a two level nested logit structure. Let’s suppose however, that our source data does not quite look like a simple random sample of travelers. Instead, suppose we have a stratified sample based on distribution channel (how travelers purchased their tickets). This stratification will manifest itself in different levels of sampling intensity across carriers due to differences in marketing practices across those carriers.

Under an MNL model formulation, because we are including a complete set of carrier-specific constants in the model, any bias in parameter estimates relating to the stratified sampling would be isolated in the carrier-specific constants, allowing the other parameter estimates to be unbiased by the sampling. However, this does not hold for more complex GEV models unless carrier-specific constants are processed outside of any GEV or nesting structure.

To do this, we can add a second set of carrier-specific constants in a special samplingbias term:

from larch.roles import P,X
m.samplingbias.ca = sum(P("carrier_bias_{}".format(i))*X("carrier={}".format(i)) for i in [2,3,4,5])


Just like the “normal” carrier-specific constants, we hold out a carrier (#1) as the reference point, and only add constants for 2 through 5.

We can now estimate the likelihood maximizing parameters:

>>> result = m.maximize_loglike('SLSQP')

>>> print(m.report('txt', sigfigs=3))
============================================================================================...
Model Parameter Estimates
--------------------------------------------------------------------------------------------...
Parameter       InitValue       FinalValue      StdError        t-Stat          NullValue
timeperiod=2     0.0             0.0901          0.00775         11.6            0.0
timeperiod=3     0.0             0.104           0.00779         13.4            0.0
timeperiod=4     0.0             0.0887          0.00853         10.4            0.0
timeperiod=5     0.0             0.127           0.00869         14.6            0.0
timeperiod=6     0.0             0.229           0.00878         26.1            0.0
timeperiod=7     0.0             0.269           0.00986         27.3            0.0
timeperiod=8     0.0             0.293           0.0103          28.3            0.0
timeperiod=9     0.0            -0.0223          0.011          -2.04            0.0
carrier=2        0.0            -0.82            0.113          -7.26            0.0
carrier=3        0.0            -2.04            0.153          -13.3            0.0
carrier=4        0.0            -1.7             0.231          -7.35            0.0
carrier=5        0.0            -6.84            8.48           -0.807           0.0
equipment=2      0.0             0.387           0.0086          45.0            0.0
fare_hy          0.0            -0.000948        2.52e-05       -37.6            0.0
fare_ly          0.0            -0.00104         7.02e-05       -14.8            0.0
elapsed_time     0.0            -0.00479         0.00011        -43.4            0.0
nb_cnxs          0.0            -2.45            0.0458         -53.5            0.0
mu_tod           1.0             0.822           0.0128         -13.9            1.0
mu_los           1.0             0.806           0.00911        -21.3            1.0
carrier_bias_2   0.0             1.13            0.138           8.23            0.0
carrier_bias_3   0.0             3.16            0.169           18.6            0.0
carrier_bias_4   0.0             2.64            0.279           9.44            0.0
carrier_bias_5   0.0             7.86            10.5            0.747           0.0
============================================================================================...
Model Estimation Statistics
--------------------------------------------------------------------------------------------...
Log Likelihood at Convergence           -777321.75
Log Likelihood at Null Parameters       -953940.44
--------------------------------------------------------------------------------------------...
Rho Squared w.r.t. Null Parameters      0.185
============================================================================================...


Tip

m = larch.Model.Example(307)