st: Efficiently run three loops (or do without them)

[Ronnie Babigumira <rb.glists@gmail.com>]

Dear list
I have quarterly data that looks like this

      qtr   houscode   fup_pdt   fup_unit   fup_qtycoll
        1        562        23          2            50
        1        570       628          2             2
        1        573       628        201            10
        1        573       628          2             2
        1        576       628        201             5
        1        576       628        201            20
        1        577       628          2             1
        1        578       628          2             1
        1        590        34         26            60
        1        595        34         26           200

For each quarter, I would like to identify "strange" values (outliers) in the variable fup_qtycoll
(simply to rule out data entry error).

This would be done for the different fup_pdt and fup_unit combinations

My initial idea is that I would have to do it in three -foreach- loops, be something along the lines
(this does not work since I need to -preserve- before -keep-ing, which I would like to avoid, and it
is not by quarter yet)

levelsof fup_pdt, local(fupdts)
foreach i of local fupdts {
		keep if fup_pdt == `i'
		levelsof fup_unit, local(fupunits)
		foreach j of local j {
			keep if fup_unit == `j'
			*** run my checks and other stuff
	    }	
}

I would appreciate some help on how I can do this efficiently

Ronnie

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