choice-metrics.com

Hi,

Consider the following case:

I have defined an alternative (ALT1) by using two attributes Travel time (TT) and Time spent in congestion (Tcong). The levels for the attributes are:
TT[0.8, 0.9, 1.0, 1.1, 1.2]
Tcong[0.3, 0.4, 0.5, 0.6, 0.7]

These levels represent the multipliers which will be multiplied with the values revealed by the respondent.

E.g. If respondent gives --> TT = 20 min. Then I wish to do the following:

ALT1
TT = 0.8*20 = 16
TCong = 0.5*16 = 5 (I multiplied the calculated TT value further with one of the levels of Tcong)


Will this operation have a negative impact during the model estimation (as the level for Tcong is effectively 0.8X0.5=0.4)?


Please advise whether is it a safe operation or should I refrain from doing this.


Thanks
Neeraj

Hi Neeraj

This sounds like a (more sophisticated) form of pivot design. See this article for a discussion of this:

Rose, J.M., Bliemer, M.C.J., Hensher, D.A. and Collins, A.T. (2008) Designing Efficient Stated Choice Experiments Involving Respondent Based Reference Alternatives, Transportation Research Part B, 42(4), 395-406.

I would suggest you simulate a data set and see what happens in this case, given that you are pivoting two attributes from a common value. I suspect you are fine given that each attribute has different levels meaning that they are not perfectly correlated, however simulations should confirm this is the case.

John

Thanks a lot sir.

Can you please explain what do you mean by simulate a dataset?

Does it mean analyzing the pilot survey data for any correlation between TT and Tcong?

Is there a way I can simulate a dataset using Ngene or some other software package?

Thanks
Neeraj

John mentions pivot designs and perhaps they are suitable.

Ngene can create relative designs for you. You first need to specify segments with reference levels (such as short distance, tt = 15 min, tcong = 5 min, and long distance, tt = 45 min, tcong = 15 min), and then you need to specify pivots. If you want multiplications you can specify percentages.

For example, you can specify tt with levels [-20%,0,50%], which multiplies a given base level (say tt = 20) with 0.8 and 1.5 to create the actual levels. For more information I refer to Section 8.2 of the Ngene manual.

Thanks a lot.

I went through the section 8.3 of the manual on Pivot designs.

The requirement for my design resembles to the one mentioned in a previous thread "Pivoted design without status quo alternative" by Arnaud Blaser on Jun 27, 2014.

I don't wish to display the status quo alternative and just want to use its info to calculate the travel time for both alternatives A and B.

Eg. if responden'ts TT = 30 mins; TT_A = 0.8*30=24 mins and TT_B=1.2*30=36 mins ... Where 0.8, ..., 1.2 are the attribute levels for travel times for both the alternatives.


Can we accomplish this objective in ngene now?

Thanks
Neeraj

No, one needs to specify the reference levels in one of the alternatives. It is on our wish list to make this more flexible in the future.