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Appropriate Use of Constant Sum Data. Joel Huber-Duke University Eric Bradlow-Wharton School Sawtooth Software Conference September 2001. Appropriate Use of Constant Sum Data. What is Constant Sum Scale data? When will CSS data work? When will it fail?

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appropriate use of constant sum data

Appropriate Use of Constant Sum Data

Joel Huber-Duke University

Eric Bradlow-Wharton School

Sawtooth Software Conference

September 2001

appropriate use of constant sum data1
Appropriate Use of Constant Sum Data
  • What is Constant Sum Scale data?
  • When will CSS data work?
  • When will it fail?
  • An analysis of Volumetric Data using both HBsum and HBreg
appropriate css usage
Appropriate CSS usage
  • When people can estimate frequency of usage in a context—as examples:
    • Soft drink choice
    • Breakfast cereals
    • Prescriptions given diagnosis
    • Multiple supplier contracts
inappropriate css usage
Inappropriate CSS usage
  • As a measure of preference strength
    • Allocate 10 points proportional to your preferences
  • As a measure of choice uncertainty
    • Indicate the probability of choosing each alternative
  • As a summary across different usage contexts
    • What proportion of beverage purchases will be Coke?
an example of conditional beverage choices
An example of conditional beverage choices
  • Drink Coke when tired
  • Drink Sprite when thirsty
  • Drink Heinekens with in-laws
  • Drink Iron City with friends
  • Drink Turning Leaf when romantic
  • Drink Ripple when depressed
alternative to constant sum
Alternative to constant sum
  • Condition choices on usage situation
    • Derive situation frequency from a separate direct question
  • Ask a single choice questions
    • Derive variability by conditioning on context, or error in choice model
analysis of volumetric choice data
Analysis of Volumetric Choice Data
  • Volume estimates among four frequently purchased non-durables
  • Each alternative defined by brand, type, size, incentive and price
  • 10 different randomized sets of alternatives
  • One fixed holdout set
  • Task: How many of each would you choose? (max=10)
people reacted differently to this task
People reacted differently to this task
  • 22% of sets produced exactly one purchase
  • 33% of the sets produced none
  • 45% chose more than one purchase
  • People differed in their likelihood to use these strategies.
two stage analysis process
Two-stage analysis process
  • Need to model both choice share and volume
  • First stage: Constant sum model with ‘none’ option
  • Second stage: Hierarchical Bayes regression with item utilities from the first stage
constant sum stage
Constant Sum Stage
  • Sawtooth’s HBSUM estimates 13 parameters for each person.
  • Model: Sums are normalized as if generated from five independent probabilistic choices
    • Choice weight =5
    • Ten tasks equivalent to 50 independent probabilistic choices
  • None is included as a fifth alternative
holdout choice accuracy
Holdout choice accuracy
  • 78% hit rate
  • Mean average error predicting choice share

2.5 share points

  • Respondents differed strongly on their use of none
hbreg predicts volume as a function of
HBreg predicts volume as a function of:
  • A constant for each individual
  • The utility of each item (from HBsum)
  • Adjusting for the utility of the set
    • Coefficient will be negative to the extent that volumes are proportional to the relative value within a set
effectiveness of dual model
Effectiveness of Dual Model
  • All coefficients significant and highly variable
  • Correlation between predicted and holdout volumes = .73
conclusions
Conclusions
  • Constant sum scale measures are mainly appropriate when frequencies are easy to estimate given a set of alternatives
  • Volumetric estimates require even more of respondents, and thus are even more rare
  • Hierarchical Bayes methods are critical for correct modeling, because of the heterogeneity in the ways people respond to the task
conclusions1
Conclusions
  • We found heterogeneity with respect to
    • The use of None
    • The average volume
    • The partworths attached to the attributes
    • The degree to which alternatives are contrasted with others in the set
  • A two-stage HB allows people with idiosyncratic processes to be represented
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