Displays a graphic that summarizes outcomes in a propensity score analysis, based on strata that have been
defined in the first Phase of a propensity score analysis (PSA). The graphic displays contributions of individual
strata to the overall effect, weighing contributions of individual strata according to the relative
sizes of the respective strata. The overall effect is plotted as a heavy dashed diagonal line that runs parallel to the identity diagonal.
Either a numeric vector containing the response of interest in a
propensity score analysis, or a three column array containing response, treatment and strata.
treatment
Binary variable of same length as response; generally 0 for 'control,' 1 for 'treatment'.
A character vector with two labels or factor with two levels are also accepted.
strata
Generally integer variable; a vector of same length as response indicating the
derived strata from estimated propensity scores. Generally 5 or 6 strata used, but function is effective
for more strata. In the case when strata are defined via unique propensity scores (as from a tree), user may wish to define strata using factor.
summary
Logical (default FALSE). If TRUE then response must have rows corresponding to number of strata;
the first two columns should contain treatment and control group sizes for each stratum, and the pair of columns
should contain the appropriate summary statistics for each statum. For example, the four summary columns might have been
generated by the strata.summary output of loess.psa.
statistic
A scalar summary of the center of the response distribution. Seen next item below. Default = "mean". Note that to generate this
statistic the full vector of responses must have been input, not summaries.
trim
Allows for a trimmed mean as outcome measure, where trim is from 0 to .5 (.5 implying median).
revc
Logical; if TRUE then X and Y axes are interchanged in plot.
confint
Logical; if TRUE adds an approximate 95% confidence interval for the mean. The interval may not be realistic
it the trim argument exceeds zero.
sw
Numerical argument (default = 0.4); extends axes on lower ends, effectively moving circles to lower left.
ne
Numerical argument (default = 0.5); extends axes on upper ends, effectively moving circles to upper right.
inc
Numerical argument (default = 0.35); controls circle sizes, but relative circle sizes
are controlled via pw. In general one wants circle areas to appear subjectively to be
sized in accordance with strata sizes.
pw
numerical argument (default = 0.4); controls relative circle sizes. pw denotes power or exponent for radius of circle.
lab
Logical (default TRUE); labels circles with stratum labels.
labcex
numerical argument (default = 1); controls the size of the circle labels.
xlab
Label for horizontal axis, by default taken from treatment.
ylab
Label for vertical axis, by default taken from treatment.
main
Main label for graph.
Details
A circle is plotted for each stratum, centered on the means for the treatment and
control groups (for the X and Y axes) respectively. The sizes of the circles
correspond to the relative sizes of the strata. A diagonal line (lower left to upper right) shows the identity,
X=Y, so that circles on, say, the lower side of this line show that the corresponding X
mean is larger than the Y mean for that stratum, and vice-versa. Parallel projections are
made from the centers of the strata-cum-circles to difference scores that are plotted on a line
segment on the lower-left corner of the graphic; the average difference, which corresponds to
the average treatment effect (ATE) for the overall treatment effect, is plotted as a
heavy (dark blue) dashed line parallel to the identity diagonal. Rug plots are shown on the upper and
right margins of the graphic, for the X and Y marginal distributions. A 95%
confidence interval for the overall effect is plotted to the left of the distribution of the
stratum difference scores, centered on the ATE. Trimmed means can replace the conventional
mean for both the ATE and the marginal distributions (however, the confidence interval
calculations are likely to become less trustworthy as larger values of the trim argument are used).
Value
Generate a Propensity Assessment Plot, as well as numerical data for
summary.strata
An array with rows corresponding to strata and four columns; these show counts for
control and treatment groups, as well as (possibly trimmed) mean response values for control and treatment.
wtd.Mn.(Name1)
Weighted mean of response for (Name1) group. Name taken from treatment.
wtd.Mn.(Name2)
Weighted mean of response for (Name2) group. Name taken from treatment.
ATE
Average Treatment Effect.
se.wtd
Weighted standard error for ATE
approx.t
Ratio of the average treatement effect and a standard error based on weighting of stratum variances.
df
Estimate of degree of freedom; response vector length minus twice number of strata.
CI.95
Approximate 95% confidence interval for overall effect size.