Many of you have asked us to provide estimation techniques for constant-sum (chip allocation) response scales rather than just analysis of "first choices" for CBC data. Constant sum questions are often used in pharmaceutical research and for some packaged goods categories. For example, it might make sense to ask buyers of breakfast cereals to indicate how many of the next ten purchases would be of each brand in a specific choice scenario (task).
For the next 10 boxes of breakfast cereal you might buy, how many would be of each if these were your only choices?
16 oz. box
12 oz. box
14 oz. box
16 oz. box
While it is true that chip-allocation tasks such as this breakfast cereal example provide more statistical information from each choice task, respondents also take more time to answer. There is also some concern about whether respondents realistically can complete the task, or whether they become distracted by the requirement that their allocations sum to some particular value. We should note, however, that our software does not require that the allocation sum to any particular value. Because we normalize the allocations within task, the researcher could remove the constant-sum requirement.
We're pleased to announce the release of a new product called HB-Sum that uses hierarchical Bayes to estimate individual-level partworth utilities from constant sum choice tasks. This product is very similar to our popular CBC/HB software. Like CBC/HB, it includes such advanced capabilities as the ability to constrain utilities to have certain signs or orders, the inclusion of linear coefficients, and the ability to estimate interaction terms. It can estimate part worths for up to 50 attributes with 25 levels each (as long as the total number of parameters to be estimated per respondent does not exceed 90). The cost is $2,000, or $1,500 for groups that already own the CBC/HB software.
We'd like to point out that our CBC Windows software cannot collect allocation data within its computer-based survey. However, one could easily use the paper-and-pencil data collection mode to collect the responses. Experienced Ci3 users could program their own randomized experiments in Ci3 that would collect allocation-based data, including on-the-fly verification that the values sum to a certain quantity, if desired. With either approach and some additional data processing, one could prepare the ASCII files needed to estimate part worth utilities using this new HB-Sum software.
Future releases of CBC products may also support constant-sum data collection. We have our concerns about the general use of constant-sum scales for CBC-type questionnaires, but we recognize that some situations may call for that approach. We are looking forward to two papers that will be presented at the 2001 Sawtooth Software Conference on the use of constant-sum conjoint experiments.