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From |
Taavi Lai <taavi.lai@ut.ee> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Newbie: Case selection problem |

Date |
Wed, 02 Mar 2005 15:55:21 +0200 |

Thanks again,

The code in the original letter worked OK, but as David points out there was a problem naming and identifying all the combinations and it didn't quite solve my problems. What I finaly came up with follows

As I don't feel myself comfortable with foreach and forvalues I did it the long way using a sample from full file.

there was ~9000 persons identified by donorid and the total number of different diagnoses was 17

gen whatever=1

collapse (sum) whatever, by(donorid)

drop whatever

save temp1.dta

save temp2.dta

append using temp1.dta /* 17 times */

sort donorid

egen count=seq(), by(donorid)

save temp2.dta

use original.dta

egen count=seq(), by(donorid)

joinby donorid count using temp2.dta, unm(b)

drop _merge

reshape wide dgn, i(donorid) j(count)

egen combination=concat( dgn_1 ... dgn_17), punct(", ")

egen combination2=ends(combination), punct(", ,") head

drop combination

tab combination2

The resulting frequency tabel is a starting point for a health state valuation workgroup which has to select most common diseases and combinations for their work. The full dataset has ~1800 different diag values, so its hard to imagine the final frequency tabel.

Best regards,

Taavi

David Kantor wrote:

This is a different problem. How many possible combinations of k potential diagnoses are there?

The answer is 2^k, and there is a natural (one-to-one) mapping from these combinations to the integers from 0 to 2^k -1. But if k is large, then how do you name all the combinations? You may be stuck with just using the resulting integer.

First, you need to have your diag values to be in a small range of non-negative integers, such as 0-k (with minimal gaps in this range). If they already are in such a form, okay. Else, you need to map them (one-to-one) into such a set of integers. (If your diag values are string, you can -encode- them and use the encoded values.)

Next, suppose that diag is that variable (or a one derived by an appropriate mapping).

summ diag

local diagmax = r(max) // get the maximal value (corresponds to k in the above)

assert r(min) >=0 // we really don't want any negatives

sort personid

forvalues n = 0 / `diagmax' {

egen byte hasdiag`n' = max(diag==`n'), by(personid)

}

/* That is like what I wrote in the previous reply -- but compacted under a -forvalues-. */

/* At this point you can condense to one observation per person; this is optional. */

bysort personid: keep if _n==1

/* Generate the identifier of all combinations. */

gen long combination = 0

forvalues n = 0 / `diagmax' {

replace combination = combination + 2^`n' if hasdiag`n'

}

----

There may be other (better) ways to express that computation.

Also, be warned, I have not tested this.

And, if `diagmax' is large, you may need double rather than long -- for the type of combination.

If this has been done correctly, then each value of combination should uniquely correspond to a distinct combination of diag values. The correspondence is that for each diag value of n, that diag value is present if and only if there is a 1 in the nth bit of the binary representation of combination (counting from the right, starting with 0) -- but only when represented as an integer (not float or double).

----

Again, I hope this helps.

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