Create 3-way percentages table - stata

Question

I would like to have a 3-way table displaying column or row percentages using three categorical variables. The command below gives the counts but I cannot find how to get percentages instead.
sysuse nlsw88
table married race collgrad, col
--------------------------------------------------------------------
| college graduate and race
| ---- not college grad ---- ------ college grad ------
married | white black other Total white black other Total
----------+---------------------------------------------------------
single | 355 256 5 616 132 53 3 188
married | 862 224 12 1,098 288 50 6 344
--------------------------------------------------------------------
How can I get percentages?

Answer 1

This answer will show a miscellany of tricks. The downside is that I don't know an easy way to get exactly what you ask. The upside is that all these tricks are easy to understand and often useful.
Let's use your example, which is excellent for the purpose.
. sysuse nlsw88, clear
(NLSW, 1988 extract)
Tip #1 You can calculate a percent variable for yourself. I focus on % single. In this data set married is binary, so I won't show the complementary percent.
Once you have calculated it, you can (a) rely on the fact that it is constant within the groups you used to define it (b) tabulate it directly. I find that tabdisp is underrated by users. It's billed as a programmer's command, but it is not difficult to use at all. tabdisp lets you set a display format on the fly; it does no harm and might be useful for other commands to assign one directly using format.
. egen pcsingle = mean(100 * (1 - married)), by(collgrad race)
. tabdisp collgrad race, c(pcsingle) format(%2.1f)
--------------------------------------
| race
college graduate | white black other
-----------------+--------------------
not college grad | 29.2 53.3 29.4
college grad | 31.4 51.5 33.3
--------------------------------------
. format pcsingle %2.1f
Tip #2 A user-written command groups offers different flexibility. groups can be installed from SSC (strictly, must be installed before you can use it). It's a wrapper for various kinds of tables, but using list as a display engine.
. * do this installation just once
. ssc inst groups
. groups collgrad race pcsingle
+-------------------------------------------------------+
| collgrad race pcsingle Freq. Percent |
|-------------------------------------------------------|
| not college grad white 29.2 1217 54.19 |
| not college grad black 53.3 480 21.37 |
| not college grad other 29.4 17 0.76 |
| college grad white 31.4 420 18.70 |
| college grad black 51.5 103 4.59 |
|-------------------------------------------------------|
| college grad other 33.3 9 0.40 |
+-------------------------------------------------------+
We can improve on that. We can set up better header text using characteristics. (In practice, these can be less constrained than variable names but often need to be shorter than variable labels.) We can use separators by calling up standard list options.
. char pcsingle[varname] "% single"
. char collgrad[varname] "college?"
. groups collgrad race pcsingle , subvarname sepby(collgrad)
+-------------------------------------------------------+
| college? race % single Freq. Percent |
|-------------------------------------------------------|
| not college grad white 29.2 1217 54.19 |
| not college grad black 53.3 480 21.37 |
| not college grad other 29.4 17 0.76 |
|-------------------------------------------------------|
| college grad white 31.4 420 18.70 |
| college grad black 51.5 103 4.59 |
| college grad other 33.3 9 0.40 |
+-------------------------------------------------------+
Tip #3 Wire display formats into a variable by making a string equivalent. I don't illustrate this fully, but I often use it when I want to combine a display of counts with numerical results with decimal places in tabdisp. format(%2.1f) and format(%3.2f) might do fine for most variables (and incidentally the important detail is the number of decimal places) but they would lead to a display of a count of 42 as 42.0 or 42.00, which would look pretty silly. The format() option of tabdisp does not reach into the string and change the contents; it doesn't even know what the string variable contains or where it came from. So, strings just get shown by tabdisp as they come, which is what you want.
. gen s_pcsingle = string(pcsingle, "%2.1f")
. char s_pcsingle[varname] "% single"
groups has an option to save what is tabulated as a fresh dataset.
Tip #4 To have a total category, temporarily double up the data. The clone of the original is relabelled as a Total category. You may need to do some extra calculations, but nothing there amounts to rocket science: a smart high school student could figure it out. Here a concrete example for line-by-line study beats lengthy explanations.
. preserve
. local Np1 = _N + 1
. expand 2
(2,246 observations created)
. replace race = 4 in `Np1'/L
(2,246 real changes made)
. label def racelbl 4 "Total", modify
. drop pcsingle
. egen pcsingle = mean(100 * (1 - married)), by(collgrad race)
. char pcsingle[varname] "% single"
. format pcsingle %2.1f
. gen istotal = race == 4
. bysort collgrad istotal: gen total = _N
. * for percents of the global total, we need to correct for doubling up
. scalar alltotal = _N/2
. * the table shows percents for college & race | collgrad and for collgrad | total
. bysort collgrad race : gen pc = 100 * cond(istotal, total/alltotal, _N/total)
. format pc %2.1f
. char pc[varname] "Percent"
. groups collgrad race pcsingle pc , show(f) subvarname sepby(collgrad istotal)
+-------------------------------------------------------+
| college? race % single Percent Freq. |
|-------------------------------------------------------|
| not college grad white 29.2 71.0 1217 |
| not college grad black 53.3 28.0 480 |
| not college grad other 29.4 1.0 17 |
|-------------------------------------------------------|
| not college grad Total 35.9 76.3 1714 |
|-------------------------------------------------------|
| college grad white 31.4 78.9 420 |
| college grad black 51.5 19.4 103 |
| college grad other 33.3 1.7 9 |
|-------------------------------------------------------|
| college grad Total 35.3 23.7 532 |
+-------------------------------------------------------+
Note the extra trick of using a variable not shown explicitly to add separator lines.