Dear all,

I would like to ask whether anyone has experience with the problem below.


I want to select a subset of the sample (see data below) so that each level
(1,2,3,4 in the example) for every variable (v1,v2,v3,v4 in the example) is
shown at least once in the subset.  I also want the sample size of the
subset to be as small as possible.  Any help on it is greatly appreciated.


    Id v1 v2 v3 v4

[1,]  1 1 2 4 3

 [2,]  2 2 1 3 4

 [3,]  3 4 2 4 2

 [4,]  4 1 1 2 3

 [5,]  5 3 2 3 4

 [6,]  6 3 1 1 1

 [7,]  7 3 4 3 1

 [8,]  8 4 4 4 4

 [9,]  9 1 2 2 1

[10,] 10 4 1 1 2

[11,] 11 2 4 3 2

[12,] 12 1 4 2 3

[13,] 13 2 3 3 4

[14,] 14 4 3 1 2

[15,] 15 3 2 1 2

[16,] 16 2 3 2 3

[17,] 17 1 4 1 4

[18,] 18 2 3 4 3

[19,] 19 4 1 4 1

[20,] 20 3 3 2 1



Thanks,

Peter

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