---
title: "Ensuring Model Feasibility"
author: "Michael Robbins and Steven Davenport"
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
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---

Using microSynth is easy when the models used to calculate weights are feasible.
But as more variables are used for matching, especially when data is scarce or
variables are sparse, the risk of an infeasible model increases. Below is a
quick guide to how to troubleshoot model feasiblity issues.

## Causes of model infeasibility

Model infeasibility becomes increasingly likely when:

* There are few control observations 
* More variables used for matching 
* Matching variables are sparse (e.g., mostly zero) 
* Treatment units have extreme values for matching variables 
* Permutation weights are calculated in addition to main weights 
* Jackknife weights are calculated in addition to main weights

## Responses to an infeasible model

As there are multiple causes of model infeasibility, there is an equally broad
range of responses.

### Specification of matching variables

If a model is found to be infeasible, the problem may trace back to matching
variable specification. We recommend the following diagnostic steps:

* Review the frequency of matching variables (e.g., with `hist()` or `table()`)
to check for sparseness. Sparse variables are difficult to match on without
large sample sizes. 
* Compare the distribution of variable values in treatment
units to the un-treated units. 
* Attempt to reduce the number of matching
variables, move variables from exact matches (`match.out`/`match.covar`) to
best-possible matches (`match.out.min`/`match.covar.min`), or aggregate
time-variant variables before matching.

When attempts to match on a sparse variable cause model infeasibility, there are
several solutions:

* Do not attempt an exact match. If the variable is time-invariant, move it from
`match.covar` to `match.covar.min`; if the variable is time-variant, move it
from `match.out` to `match.out.min`. 
* If the variable is time-variant,
aggregate the variable over multiple time periods before matching. If just one
or several variables that appear to be sparse or for which the treatment
contains values that are rare in the un-treated units, then the user can issue
instructions for each of those variables to be aggregated over different time
periods. (Those time periods do not have to be at regular intervals, for
instance if the sparseness only occurs at certain points in the pre-intervention
data.) Exercise 4 from the \link{Introduction} provides an example of this. If
the user would like to aggregate all time-variant variables over the same
regular time periods, then it is somewhat simple to pass `match.out` or
`match.out.min` a vector of variable names, and specify the aggregation periods
using `period`.

### Parameters for calculating weights

If varying the specification of matching variables is not satisfactory, the user
can set the parameters microSynth() uses for the calculation of weights.

* `max.mse` may be raised. This relaxes the constraint governing matches for
variables passed to `match.out` and `match.covar`. 
* Advanced users may wish to
alter `maxit`, `cal.epsilon`, `calfun`, and `bounds`, which correspond to
parameters from the `survey::calibrate()` and govern the calculation of weights.

### Calling on (computationally-intensive) back-up models

By default microSynth() attempts to calculate weights using simple methods. But
because these are not always sufficient to produce a feasible model, two
arguments, `check.feas` and `use.backup`, specify how microsynth should find and
use less restrictive backup models. The two arguments do not interact and can be
set independently.

`check.feas = TRUE` will search for a single model that yields satisfactory
constraints for all purposes: estimating main weights, permutation weights, and
jackknife residuals. The same model will be used for all purposes.

`use.backup = TRUE` will calculate the main weights without checking for
feasibility, but if weights appear to be poor (i.e., they do not satisfy the
max.mse condition), then weights will be re-calculated using another model. This
way, different backup models may be used for different purposes (i.e., for
estimating main weights, permutation weights, and jackknife residuals).
