Add a column of differences to tables of summary statistics in Stata

Question

If I make a two way summary statistics table in Stata using table, can I add another column that is the difference of two other columns?

Say that I have three variables (a, b, c). I generate quintiles on a and b then generate a two-way table of means of c in each quintile-quintile intersection. I would like to generate a sixth column that is the difference of mean c between the top and bottom quintiles of b for each quintile of a.

I can generate the table of mean c for each quintile-quintile intersection, but I can't figure out the difference column.

* generate data
clear
set obs 2000
generate a = rnormal()
generate b = rnormal()
generate c = rnormal()

* generate quantiles for for a and b
xtile a_q = a, nquantiles(5)
xtile b_q = b, nquantiles(5)

* calculate the means of each quintile intersection
table a_q b_q, c(mean c)

* if I want the top and bottom b quantiles
table a_q b_q if b_q == 1 | b_q == 5, c(mean c)

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Update: Here's an example of what I would like to do.

Answer 1

With the collapse command you can create customized tables like the one you have in mind.

preserve
collapse (mean) c, by(a_q b_q)
keep if inlist(b_q, 1, 5)
reshape wide c, i(a_q) j(b_q)
gen c5_c1 = c5 - c1
set obs `=_N + 1'
replace c1 = c1[`=_N - 1'] - c1[1] if mi(a_q)
replace c5 = c5[`=_N - 1'] - c5[1] if mi(a_q)
replace c5_c1 = c5_c1[`=_N - 1'] - c5_c1[1] if mi(a_q)
list, sep(0) noobs
restore

Then you should obtain something like this in your output:

  +-----------------------------------------+
  | a_q          c1          c5       c5_c1 |
  |-----------------------------------------|
  |   1    .2092651    .1837719   -.0254932 |
  |   2    .0256483   -.0118134   -.0374617 |
  |   3     .022957    .0586441    .0356871 |
  |   4    .0431809    .0876745    .0444935 |
  |   5   -.0859874    .0199202    .1059076 |
  |   .   -.2952525   -.1638517    .1314008 |
  +-----------------------------------------+

If you are not very familiar with Stata, the following help pages might be useful in understanding the code

help _variables
help subscripting