The PSMATCH Procedure
STRATA Statement
STRATA <options>;
The STRATA statement divides observations in the support region into strata based on propensity scores, where the support region is specified in the REGION= option in the PROC PSMATCH statement. The STRATA statement does not apply when you specify the MATCH statement.
Table 7 summarizes the options in the STRATA statement.
Table 7: STRATA Statement Options
| Option | Description |
|---|---|
| KEY= | Specifies how to use the observations to construct the strata |
| NSTRATA= | Specifies the number of strata |
| STRATUMWGT= | Specifies the type of stratum weights to combine statistics across strata |
- KEY=TOTAL | TREATED
-
specifies how to use the observations to construct the strata.
- TOTAL
requests that each stratum contain approximately the same number of total units, which can be in either the treated group or the control group.
- TREATED
requests that each stratum contain approximately the same number of units in the treated group.
By default, KEY=TREATED. This option balances the number of treated units across strata, so that a reliable estimate of the treatment effect can be obtained for each stratum. However, a common alternative is to construct strata so that each stratum has the same number of total units. You can request this approach by specifying KEY=TOTAL. In either case, you should examine the number of treated units and the number of control units in each stratum to make sure that a reliable estimate can be obtained for each stratum.
- NSTRATA=n
specifies the number of strata, where n
2.
Only observations in the support region are stratified. By default, NSTRATA=5. -
STRATUMWGT=TOTAL | TREATED
STRATUMWEIGHT=TOTAL | TREATED -
specifies the type of stratum weights that the PSMATCH procedure uses to combine stratum-specific statistics in the assessment of variable balance after stratification. For more information about stratum weights, see the section Weighting after Stratification.
- TOTAL
uses the proportions of total units (treated and control units combined) in the strata as the weights. These weights sum up to one and are appropriate for estimating the ATE.
- TREATED
uses the proportions of treated units in the strata as the weights. These weights sum up to one and are appropriate for estimating the ATT.
By default, STRATUMWGT=TOTAL.
For more information, see the section Propensity Score Stratification.