Tutorial¶
We now illustrate the basic capabilities of the grmpy package. We start with the assumptions about functional form and the distribution of unobservables and then turn to some simple use cases.
Assumptions¶
The grmpy package implements the normal linear-in-parameters version of the generalized Roy model. Both potential outcomes and the cost associated with treatment participations \((Y_1, Y_0, C)\) are a linear function of the individual’s observables \((X, Z)\) and random components \((U_1, U_0, U_C)\).
We collect all unobservables determining treatment choice in \(V = U_C - (U_1 - U_0)\). The unobservables follow a normal distribution \((U_1, U_0, V) \sim \mathcal{N}(0, \Sigma)\) with mean zero and covariance matrix \(\Sigma\). Individuals decide to select into treatment if their surplus from doing so is positive \(S = Y_1 - Y_0 - C\). Depending on their decision, we either observe \(Y_1\) or \(Y_0\).
Model Specification¶
You can specify the details of the model in an initialization file (example). This file contains several blocks:
SIMULATION
The SIMULATION block contains some basic information about the simulation request.
| Key | Value | Interpretation |
|---|---|---|
| agents | int | number of individuals |
| seed | int | seed for the specific simulation |
| source | str | specified name for the simulation output files |
ESTIMATION
The ESTIMATION block determines the basic information for the estimation process.
TREATED
The TREATED block specifies the number and order of the covariates determining the potential outcome in the treated state and the values for the coefficients \(\beta_1\).
| Key | Column | Value | Interpretation |
|---|---|---|---|
| coeff | str | float | intercept coefficient |
| coeff | str | float | coefficient of the first covariate |
| coeff | str | float | coefficient of the second covariate |
UNTREATED
The UNTREATED block specifies the number and order of the covariates determining the potential outcome in the untreated state and the values for the coefficients \(\beta_0\). In particular, the integer in the column Column specifies the column in the relevant dataset.
COST
The COST block specifies the number and order of the covariates determining the cost of treatment and the values for the coefficients \(\gamma\). In particular, the integer in the column Column specifies the column in the relevant dataset.
| Key | Column | Value | Interpretation |
|---|---|---|---|
| coeff | str | float | intercept coefficient |
| coeff | str | float | coefficient of the first covariate |
| coeff | str | float | coefficient of the second covariate |
DIST
The DIST block specifies the distribution of the unobservables.
| Key | Value | Interpretation |
|---|---|---|
| coeff | float | \(\sigma_{U_1}\) |
| coeff | float | \(\sigma_{U_1,U_0}\) |
| coeff | float | \(\sigma_{U_1,V}\) |
| coeff | float | \(\sigma_{U_0}\) |
| coeff | float | \(\sigma_{U_0,V}\) |
| coeff | float | \(\sigma_{V}\) |
SCIPY-BFGS
The SCIPY-BFGS block contains the specifications for the BFGS minimization algorithm. For more information see: SciPy documentation.
| Key | Value | Interpretation |
|---|---|---|
| gtol | float | the value that has to be larger as the gradient norm before successful termination |
| eps | float | value of step size (if jac is approximated) |
SCIPY-POWELL
The SCIPY-POWELL block contains the specifications for the POWELL minimization algorithm. For more information see: SciPy documentation.
| Key | Value | Interpretation |
|---|---|---|
| xtol | float | relative error in solution values xopt that is acceptable for convergence |
| ftol | float | relative error in fun(xopt) that is acceptable for convergence |
Examples¶
In the following chapter we explore the basic features of the grmpy package. The resources for the tutorial are also available online.
So far the package provides the features to simulate a sample from the generalized roy model and to estimate the parameters of interest (given a data set) as specified in your initialization file.
Simulation
First we will take a look on the simulation feature. For simulating a sample from the generalized roy model you use the simulate function provided by the package. For simulating a sample of your choice you have to provide the path of your initalization file as an input to the function.
import grmpy
grmpy.simulate('tutorial.grmpy.ini')
This creates a number of output files that contain information about the resulting simulated sample.
- data.grmpy.info, basic information about the simulated sample
- data.grmpy.txt, simulated sample in a simple text file
- data.grmpy.pkl, simulated sample as a pandas data frame
Estimation
The other feature of the package is the estimation of the parameters of interest. The specification regarding start values and and the optimizer options are determined in the ESTIMATION section of the initialization file.
grmpy.estimate('tutorial.grmpy.ini')
As in the simulation process this creates a number of output file that contains information about the estimation results.
- est.grmpy.info, basic information of the estimation process
- comparison.grmpy.txt, distributional characteristics of the input sample and the samples simulated from the start and result values of the estimation process