Additional Arguments for Nearest Neighbor Matching
m.order: the order in which to match treatment units
with control units.
"largest" (default): matches from the largest value of
the distance measure to the smallest.
"smallest": matches from the smallest value of the
distance measure to the largest.
"random": matches in random order.
replace: logical value indicating whether each control
unit can be matched to more than one treated unit (default = replace = FALSE, each control unit is used at most once - i.e.,
sampling without replacement). For matching with replacement, use
replace = TRUE. After matching with replacement, the weights
can be used to reflect the frequency with which each control unit was matched.
ratio: the number of control units to match to each
treated unit (default = 1). If matching is done without
replacement and there are fewer control units than ratio times
the number of eligible treated units (i.e., there are not enough
control units for the specified method), then the higher ratios will
have NA in place of the matching unit number in
match.matrix.
exact: variables on which to perform exact matching
within the nearest neighbor matching (default = NULL, no exact
matching). If exact is specified, only matches that
exactly match on the covariates in exact will be allowed.
Within the matches that match on the variables in exact,
the match with the closest distance measure will be chosen.
exact should be entered as a vector of variable names
(e.g., exact = c("X1", "X2")).
caliper: the number of standard deviations of the
distance measure within which to draw control units (default = 0, no caliper matching). If a caliper is specified, a control
unit within the caliper for a treated unit is randomly selected as
the match for that treated unit. If caliper != 0, there
are two additional options:
calclosest: whether to take the nearest available
match if no matches are available within the caliper
(default = FALSE).
mahvars: variables on which to perform
Mahalanobis-metric matching within each caliper (default = NULL). Variables should be entered as a vector of variable
names (e.g., mahvars = c("X1", "X2")). If
mahvars is specified without caliper, the
caliper is set to 0.25.
subclass and sub.by: See the options for
subclassification for more details on these options. If a
subclass is specified within method = "nearest",
the matched units will be placed into subclasses after the nearest
neighbor matching is completed.
