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CBC: Include one particular alternative in each choice set

I have used SSI Web v7 to make an alternative-specific CBC design, which is now all set up and running. My choice tasks have 3 alternatives. My primary attribute has 4 levels and a status quo level, so 5 levels in total. I want the status quo level to always be one of the three alternatives in a choice set. How can I do this?

Another issue leading on from this is that I just want my primary attribute to be constant- the other attributes for the status quo alternative aren't always constant.

For example: the primary attribute is car type. I want "diesel" to come up in each choice task as one of the alternatives, but I do want to have attribute levels for the 'car price' attribute, which is not necessarily constant for "diesel".

Any help is much appreciated!
asked Aug 13, 2015 by Beau

1 Answer

0 votes
Our design algorithms use goals in coming up with what concepts to show.  One of these goals is that levels should be used an equal number of times.  What you want to happen goes against this goal as it will oversample the status quo level.  Unfortunately there isn't an easy way in the software to do this.  However, I can think of two approaches:

1) In general I think the only way to accommodate it would be to put in prohibitions to prevent the combinations you don't want.  This would be lots of cross-concept prohibitions (don't allow A/B/C, A/C/D, etc.), and would require the Advanced Design Module.

2) However, because it is an alternative-specific setup (meaning it sounds like no other attributes apply to the status quo level), then you could probably  generate a design with the original 4 levels and the design set to only show 3 concepts.  Export that design to Excel, and the modify it by hand to add a fourth concept to each task that is level 5, and then zeroes for all the other attributes.  Add level 5 back in to your design and then import your modified design.

Make sure to use the Test Design routine to ensure that this deviation from efficiency isn't causing too much loss of information from your other levels.
answered Aug 13, 2015 by Brian McEwan Gold Sawtooth Software, Inc. (38,865 points)