## histover

A stata program that plots multiple superimposed distributions.

**Installation **

Install histover by opening Stata and running the command:

net install histover, from(http://kylerozema.com/histover)

The documentation for histover is included with the installation. To read the documentation, run the command:

help histover

**Background**

Suppose we want to compare two distributions. Stata’s command hist can be used to make the below graph that is from *The Political Ideologies of Law Clerks* 19 American Law and Economics Review 97 (2017). The graph plots the distributions of law clerk ideology separately for male clerks and female clerks. The x-axis is a measure of ideology known as the Campaign Finance score (“CFscore”), which situates individuals on a unidimensional ideological scale from extremely liberal to extremely conservative. As explained in the article from which the example is drawn, “[t]he scale is normalized such that it has a mean of zero and a standard deviation of one with respect to the population of U.S. donors. For example, Barack Obama and Hillary Clinton, on the ideological left side of the spectrum, have CFscores of -1.65 and -1.16, respectively; Joseph Lieberman and Chris Christie, ideologically more moderate, have CFscores of -0.54 and 0.46, respectively; and Scott Walker and Ron Paul, on the ideological right, have CFscores of 1.28 and 1.57, respectively.”

While this side-by-side comparison is useful, it is still difficult to visually compare the distributions. We can easily stack the histograms vertically but comparisons of the distributions are still difficult. A better way to compare distributions is to superimpose them. One can plot multiple superimposed kernel density plots (in stata, if you want to plot the distribution of y separately by x=0 and x=1, one uses twoway (kdensity y if x == 0) || (kdensity y if x == 1)), but the smoother might not be preferred. I often superimpose distributions in my research directly without kernel density estimation to make comparisons of the distributions easier. I wrote a stata command to help other’s make similar superimposed distributions as well. Since I have written the command, stata has come out with options to change the transparency in graphs that allows for superimposed histograms (read about it here).

**Technical Description**

The plot below is made using histover. The syntax takes the form: histover *var_outcome*, by(*var_unique*) bmin(*min*) bmax(*max*) interval(*step*), where *var_outcome* is the outcome variable (the distribution that will be plotted), *var_unique* is variable that defines each of the distributions to be plotted, *min* is the user selected minimum that will be plotted in the distributions, *max* is the user selected maximum that will be plotted in the distributions, and *step* is the user selected step or interval that will be used to define the bins in the distributions.