bal.ms.psa {PSAgraphics}

Balance for Continuous Covariate: Random Strata as part of a PSA

Description

Function provides a measure (based on the trimmed mean) of the balance achieved between control and treatment groups for a continuous covariate from user defined strata. This statistic is compared to the same measure for randomly permuted strata.

Usage

bal.ms.psa(
  continuous,
  treatment = NULL,
  strata = NULL,
  trim = 0,
  B = 1000,
  main = NULL
)

Arguments

continuous

Quantitative covariate that is being balanced within strata in a PSA. If continuous has three columns, then the second and third are assumed to be the treatment and strata respectively. Missing values are not allowed.

treatment

Binary variable of same length as continuous; generally 0 for 'control,' 1 for 'treatment.'

strata

Integer variable; a vector of same length as continuous indicating the derived strata from estimated propensity scores.

trim

Fraction (0 to 0.5) of observations to be trimmed from each end of stratum-treatment level before the mean is computed. See mean.

B

Numeric; number of randomly generated iterations of the balance measure are created for the comparison distribution.

main

Title passed to histogram.

Details

This function measures the balance achieved across K strata for a continuous covariate. If \mu_{ik} is the covariate trimmed (as specified by user) mean of cases in stratum k, treatment i, then the statistic is the sum over all K of |\mu_{0k} - \mu_{1k}| . A permutation distribution is generated by randomly assigning cases to strata, thus generating B permuted stratifications and the associated B permutation statistics. The permutation stratifications are generated under a fixed marginals model to retain comparability with the original stratification. A histogram of the permutation statistics is produced with the original statistic referenced as a red dot.

Value

In addition to the histogram, a list with the following components is returned:

balance.orig

Balance measure of user defined strata.

rank.orig

Rank of original balance measure in comparison with the B randomly generated values.

Author(s)

James E. Helmreich [e-mail]

Robert M. Pruzek [e-mail]

See Also

bal.ks.psa, bal.cws.psa, bal.cs.psa

Examples

#Balance stat should be close to zero
meas<-rnorm(500)
continuous<-c(meas,meas+rnorm(500,0,.1))
treatment<-c(rep(0,500),rep(1,500))
strata<-rep(c(rep(1,100),rep(2,100),rep(3,100),rep(4,100),rep(5,100)),2)
bal.ms.psa(continuous,treatment,strata)


#Balance stat should be close to .4
meas<-rnorm(500)
continuous<-c(meas, meas[1:250] + runif(250,0,.2),
   meas[251:500]-runif(250,0,.2))
treatment<-c(rep(0,500),rep(1,500))
strata<-rep(c(rep(1,100), rep(2,100), rep(3,100),
   rep(4,100),rep(5,100)),2)
bal.ms.psa(continuous, treatment, strata, B=200)