by **munshi.nawaz** » Sun Dec 31, 2017 4:34 pm

Dear John and Michiel

I am very grateful that you took the time and answered my questions in such details. I am a beginner and am undertaking three more choice analyses as part of my overall research, and I found your answers very helpful clarifying my understanding of the DCE concept. Thank you.

From your answers and from the information I found recently from this forum, I can readily see that I have potentially made an error in my design by “NOT Including a Constant” in utility functions that is recommended for labelled alternatives. I would like to take this opportunity to copy both of my initial and recent NGene Syntaxes and their relative results for you to have a quick glimpse at and seek response to some further queries listed at the end.

Initial Syntax (S1) – Priors were assumed and values were distributed in a way that adds up to 1 for each alternative. (I had started my survey based on this design.)

Design

?Trial

;Alts = Car,Improved Bus

;rows = 12

;Eff =(mnl,s)

;model:

U(Car)=A[-0.55]*TTCar[45,48,51,54]+B[-0.45]*AEWCar[0,5,10,15]/

U(Improved Bus )=C[-0.35]*TTImproved Bus[39,42,45,48]+D[-0.20]*SFImproved Bus[6,10]+E[-0.25]*AEWImproved Bus[5,10,15,20]+D*TWDImproved Bus[0,5,10]

$

MNL efficiency measures

D error 0.528789

A error 3.047175

B estimate 7.850689

S estimate 145.1522

Prior a b c d e

Fixed prior value -0.55 -0.45 -0.35 -0.2 -0.25

Sp estimates 47.5106 61.1318 145.1522 123.214 145.1086

Sp t-ratios 0.284355 0.250682 0.162684 0.176574 0.162708

Current Syntax (S2)- Priors were adjusted from the coeficients observed from the analysis of 18 survey data using NLogit.

Design

?Trial

;Alts = Car,Improved Bus

;rows = 12

;Eff =(mnl,s)

;model:

U(Car)=A[-0.15]*TTCar[45,48,51,54]+B[-0.35]*AEWCar[0,5,10,15]/

U(Improved Bus)=A*TTImproved Bus[39,42,45,48]+C[-0.25]*SFImproved Bus[6,10]+B*AEWImproved Bus[5,10,15,20]+C*TWDImproved Bus[0,5,10]

$

MNL efficiency measures

D error 0.054059

A error 0.603817

B estimate 55.11679

S estimate 38.53202

Prior a b c d e f

Fixed prior value -0.3 -0.7 -0.2 -0.1 -0.25 -0.45

Sp estimates 16.9997 13.57093 28.89348 38.53202 23.38713 13.50741

Sp t-ratios 0.475374 0.532049 0.364633 0.315751 0.405292 0.533298

Please note that

A. It has been my understanding that the priors for the same attributes could differ for different alternatives;

B. In Syntax S2, the coefficient values from the survey data did vary (signs were same), however, I have adjusted the priors in accordance with the overall ratio that was found from the MNL results.

Based on the above, could you please comment on the following:

Q1 – How important is the value of a constant in a design such as mine? As I have not added a constant in my syntax would the results be now considered as incorrect?

Q2 – Is there any way that I can adjust the design to accommodate the “constant” if it is important? Or do I need to go through the experiment process all over again?

Q3 – Referring to the notes above (A and B), could you please advise if my understanding of the priors is correct, i.e. I have used it with some degree of flexibility based on the level of assumed influences of the attributes on someone’s choice making decision for a particular alternative?

Thank you once again for the advice you both have provided.

Wish you a very happy new year.

Kind regards

Munshi