Hello, this question is for clarifying a few doubts about a DCE on participants' WTP for a new technology for EV cars for my project....and I will be so grateful to receive some initial support on this as I am new to Choice studies.
1)For the initial ethics approval we need to state the sample size (minimum) and the formula for it. I am thinking of using the Orne's rule of thumb N=500L/S*A. I am using this because true sample estimate can be found only using priors from pilot and without ethics approval we cannot get there yet. Is it fine?
2)But even to apply this formula, I need the number of choice tasks required. My alternatives are right now two, with 7 attributes which have 2,3,4,5 levels across them. As per the rule of thumb for attribute level balance, the least common multiple is 60 for (2,3,4,5) , so can I use 60 as my number of choice tasks? Is it not too much? And using this leads to only >20 sample size through Orne rule, which looks unreasonable. If you suggest blocking then how?
3)On the other hand, for number of choice tasks, if I use the formula S(J-1) >= K, then I need to know the number of parameters. Can I get a help on this parameter calculation when variables are a mix of dummy and continuous? e.g. I have 7 attributes, 6 of which are categorical or yes/no dummy and just 1 is a continuous variable. The following are the levels of these attributes (where Only Attribute-2 is continuous) :
A=Attrib-1= 3 levels
B=Attrib-2 (continuous) = 3 levels
C=Attrib-3= 4 levels
D=Attrib-4= 5 levels
E=Attrib-5= 4 levels
F=Attrib-6= 4 levels
G=Attrib-7= 2 levels
So am I right in finding num of parameters, k= 19 from above? Since we are supposed to do (L-1) for every categorical attribute as there is a base.
Using 19 as S in Orne rule gives me around 66 as minimum sample size.
3) I tried to put two simple utility functions for two alternatives with these attributes above in ngene, but it gives ''no design found'' error when i mention orthogonal design in the command. Does it mean an orthogonal design cannot be formed with these set of info?
4) In Ngene we need to mention ''rows'' , how exactly do we ascertain how many rows?
5) I have a lot of socio-demo variables as well, am I supposed to include those also in Utility functions to be fed into NGENE? Will these extra variables affect the number of parameters and therefore number of choice tasks and sample size?
6)For efficient design we are supposed to write (-0.00001) or (0.00001) in ngene command, but what if for a few categorical neutral-ish attributes we do not really know whether they positively affect decision or negatively? for example, One Attribute=Location of petrol station: Near workplace, En-route, Destination. How to give a =ve or -ve sign for such an attribute? Can we leave it just like that with no prior brackets at all in our command? Although the other attributes maybe we know the signs for and have put brackets, except this one.
7) Regarding finding the number of alternatives, is there any way or rule? Given we have one new technology whose WTP we want to find, that makes it one alternative. So all the past techs can be Alternative 2? or can we provide more alternatives? Any suggestion or reference, on this will be really helpful to me.
Thanks
Below is an example of the commands that I am toggling with for my model discussed above. Here A,B,C .... are the attributes where only B is continuous, rest are dummy /categorical:
design
;alts= Tech1*,Tech2*
;rows =19
;eff = (mnl,d)
;model:
U(Tech1) = b1 [0.00001]*A[0,1,2]
+ b2[-0.00001] *B[25,57,80]
+b3[0.00001] *C[0,1,2,3]
+ b4[0.00001] *D[0,1,2,3,4]
+ b5[-0.00001] *E[0,1,2,3]
+ b6[0.00001] *F[0,1,2,3]
+b7[-0.00001]*G[0,1] /
U(Tech2) = b1* A+ b2* B + b3* C + b4* D + b5* E + b6* F+b7* G
$