Hello,
I am designing an unlabeled CE with two product alternatives and an “opt-out” alternative. I am currently working on an efficient design, which will be used in a pilot study to generate the priors for the final Bayesian design. In the efficient design I am not going to define priors for the attributes (expect for the price attribute), since from the literature it is not clear how the levels of these attributes effect consumers’ preferences for the product in question. Below is the code I used:
design
;alts = Alt1*, Alt2*, Alt3
;rows = 12
;eff = (mnl,d)
;model:
U(Alt1) = b1.dummy[0|0]*loc[2,1,0]+b2.dummy[0|0|0]*hops[3,2,1,0]+b3.dummy[0]*org[1,0]+b4[-0.00001]*price[7.99,8.99,9.99,10.99,11.99]/
U(Alt2) = b1.dummy*loc +b2.dummy*hops +b3.dummy*org +b4*price
$
This is the first time I create an efficient design with Ngene. The Ngene manual and this forum helped me a lot, but I have a few questions:
1) As soon as I run the code, I get a warning: “One or more attributes will not have level balance with the number of rows specified: alt1.price, alt2.price”. As far as I know, attribute level balance requirement can be relaxed in the case of efficient designs. However, I was wondering whether attribute level unbalance could effect the efficiency of my priors.
2) After a few minutes, I obtain a design with a low D-error, but with an S-estimate higher than 4000000000. Is this value of the S-estimate due to the use of none or very small priors? I tried to use bigger priors and I had much better S-estimates.
3) I obtain the most “ D-efficient” design using a number of rows equal to 12, instead of 10. However, before obtaining the 12 rows design, I had this message: “A valid initial random design could not be generated after approximately 10 seconds. In this time, of the XXXXXX attempts made, there were 0 row repetitions, XXXX alternative repetitions, and XXXXXX cases of dominance. There are a number of possible causes for this, including the specification of too many constraints, not having enough attributes or attribute levels for the number of rows required, and the use of too many scenario attributes. A design may yet be found, and the search will continue for 10 minutes. Alternatively, you can stop the run and alter the syntax”.
Does this message suggest there is a problem with the 12 rows design?
I thank you in advance for you kind attention and availability.
Claudia