Empirical quantile-quantile plot: This graph plots covariate
values that fall in (approximately) the same quantile of treated and
control distributions. Control unit quantile values are plotted on
the x-axis, and treated unit quantile values are plotted on the
y-axis. If values fall below the 45 degree line, control units
generally take lower values of the covariate. Data points that fall
exactly on the 45 degree line indicate that the marginal
distributions are identical.
Jitter plots: This graph plots jittered estimated propensity
scores of treated and control units. Dark diamonds indicate matched
units and grey diamonds indicate unmatched or discarded units. The
area of the diamond is proportional to the weights. Vertical lines
are plotted if subclassification is used.
Histograms: This graph plots histograms of the estimated propensity
scores in the original treated and control groups and weighted histograms of the estimated
propensity scores in the matched treated and control groups. Plots can be compared vertically to
quickly check the balance before and after matching.
