Originally posted by Arne Risa Hole
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Dear Arne,
First, thank you very much for the very helpful dcreate module!
I encouter a curious situation where two choice sets come out identical when I attempt to generate a specific design (similar to Example 3 in the help file, with the additional optout alternative with associated ASC).
Here is my code:
Code:matrix levmat = 3,2,2 genfact, levels(levmat) matrix optout = J(1,3,1) matrix b = J(1,9,0) dcreate i.x1##i.x2 i.x1##i.x3, nalt(2) nset(30) fixedalt(optout) asc(3) bmat(b)
When the algorithm converges (after 3 iterations), choice sets #5 and #7 are exactly the same. Is this normal/to be expected?
Thanks again,
Claudio
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Claudio
Yes, with simple designs (few attributes/levels) with many choice sets this can happen  there is nothing in the dcreate code that explicitly prevents it. Note that in your case you can only construct 144 different choice sets, and some of them will not convey any information (for example a choice set with two identical alternatives), so will not be selected by dcreate.
Arne
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Originally posted by Arne Risa Hole View PostClaudio
Yes, with simple designs (few attributes/levels) with many choice sets this can happen  there is nothing in the dcreate code that explicitly prevents it. Note that in your case you can only construct 144 different choice sets, and some of them will not convey any information (for example a choice set with two identical alternatives), so will not be selected by dcreate.
Arne
Too bad there is nothing in the dcreate code to prevent generating duplicate choice sets.
I know that 66 unique 2alternative choice sets can be constructed in my case (12*11/2). I admit that I haven't investigated how the modified Fedorov algorithm works, but I didn't expect to have duplicates using the dcreate module with fewer choice sets than this number.
For your information, after playing a bit with the command by changing the number in nset to 24 and 36, and examining the results explicitly for choice sets containing the same two alternatives (either as alt=1 or alt=2 in the choice set), I obtained these results:
 With nset(24): 4 sets were duplicated twice ; 2 sets were duplicated thrice.
 With nset(36): 1 set was duplicated thrice ; 3 sets were duplicated 4 times ; and 2 sets were duplicated 5 times.
If I understand correctly, this would mean that 8 (4*1 + 2*2) and 19 (1*2 + 3*3 + 2*4) choice sets, respectively, do not add any new information over the remaining unique sets in these generated designs, translating into 33% (8/24) and 53% (19/36) wasted choice sets (again, respectively).
In such cases, would you recommend removing the duplicate choice sets or trying to generate an efficient design with fewer choice sets, or something else entirely?
Thank you again,
Claudio
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Originally posted by Arne Risa Hole View PostClaudio
I personally don't really see the issue here  I would just generate a design with fewer choice sets. Both 24 and 36 are way more than you need, and the repeated choice sets reflect that.
Arne
Claudio
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Originally posted by Arne Risa Hole View PostDear Mariya,
I am glad to hear that you have found dcreate useful. A design with a higher Defficiency is more efficient than a design for the same experiment with a lower Defficiency, but the numbers themselves don't mean much as they depend on things like the number of attributes, levels etc. in your choice experiment.
Arne
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