An exact match is one based on a factor. Within a level, all observations are allowed to be matched. An exact match can be combined with another distance matrix to create a set of matching subproblems.

## Usage

```
exactMatch(x, ...)
# S4 method for class 'vector'
exactMatch(x, treatment)
# S4 method for class 'formula'
exactMatch(x, data = NULL, subset = NULL, na.action = NULL, ...)
```

## Arguments

- x
A factor vector or a formula, used to select method.

- ...
Additional arguments for methods.

- treatment
A logical or binary vector the same length as

`x`

indicating treatment and control for each unit in the study. TRUE or 1 represents a treatment unit, FALSE or 0 represents a control unit. NA units are excluded.- data
A

`data.frame`

or`matrix`

that contains the variables used in the formula`x`

.- subset
an optional vector specifying a subset of observations to be used

- na.action
A function which indicates what should happen when the data contain

`NA`

s

## Value

A matrix like object, which is suitable to be given as
`distance`

argument to `fullmatch`

or
`pairmatch`

. The exact match will be only zeros and
`Inf`

values, indicating a possible match or no possible
match, respectively. It can be added to a another distance matrix
to create a subclassed matching problem.

## Details

`exactMatch`

creates a block diagonal matrix of 0s and
`Inf`

s. The pairs with 0 entries are within the same level of
the factor and legitimate matches. `Inf`

indicates units in
different levels. `exactMatch`

replaces the
`structure.fmla`

argument to several functions in previous
versions of optmatch. For the `factor`

method, the two
vectors `x`

and `treatment`

must be the same length. The
vector `x`

is interpreted as indicating the grouping factors
for the data, and the vector `treatment`

indicates whether a
unit is in the treatment or control groups. At least one of these
two vectors must have names. For the `formula`

method, the
`data`

argument may be omitted, in which case the method
attempts to find the variables in the environment from which the
function was called. This behavior, and the arguments `subset`

and `na.action`

, mimics the behavior of `lm`

.

## Examples

```
data(nuclearplants)
### First generate a standard propensity score
ppty <- glm(pr~.-(pr+cost), family = binomial(), data = nuclearplants)
ppty.distances <- match_on(ppty)
### Only allow matches within the partial turn key plants
pt.em <- exactMatch(pr ~ pt, data = nuclearplants)
as.matrix(pt.em)
#> control
#> treated H I J K L M N O P Q R S T U V W X Y
#> A 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> C 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> D 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> E 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> F 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> a Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf
#> b Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf
#> c Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf Inf
#> control
#> treated Z d e f
#> A 0 Inf Inf Inf
#> B 0 Inf Inf Inf
#> C 0 Inf Inf Inf
#> D 0 Inf Inf Inf
#> E 0 Inf Inf Inf
#> F 0 Inf Inf Inf
#> G 0 Inf Inf Inf
#> a Inf 0 0 0
#> b Inf 0 0 0
#> c Inf 0 0 0
### Blunt matches:
match.pt.em <- fullmatch(pt.em)
#> Warning: Without 'data' argument the order of the match is not guaranteed
#> to be the same as your original data.
print(match.pt.em, grouped = TRUE)
#> Group Members
#> 0.1 A, Z
#> 0.2 B, Y
#> 0.3 C, X
#> 0.4 D, W
#> 0.5 E, V
#> 0.6 F, U
#> 0.7 G, H, I, J, K, L, M, N, O, P, Q, R, S, T
#> 1.1 a, f
#> 1.2 b, e
#> 1.3 c, d
### Combine the propensity scores with the subclasses:
match.ppty.em <- fullmatch(ppty.distances + pt.em)
#> Warning: Without 'data' argument the order of the match is not guaranteed
#> to be the same as your original data.
print(match.ppty.em, grouped = TRUE)
#> Group Members
#> 0.1 A, D, F, V
#> 0.2 B, G, U
#> 0.3 C, H, J, K, L, M, N, O, P, Q, R, T, W, X, Y, Z
#> 0.5 E, I, S
#> 1.1 a, d
#> 1.2 b, f
#> 1.3 c, e
```