Example of a MaxDiff Design
You are the purchaser for your company’s office supplies and you need to buy candy for a holiday party. First, you want to determine which candy types people prefer. To figure this out, you conduct a MaxDiff study.
You ask five randomly chosen associates to rate seven types of candies. Based on your experience with previous studies, you realize that it is difficult for raters to rank seven types of items in order of preference. Instead, you create a design that consists of choice sets of size four. To keep the study manageable, you structure the survey as a MaxDiff study: you ask each associate to specify his or her most preferred and least preferred candy in each of the seven choice sets. (These selections result in a balanced incomplete block design. See Cochran and Cox (1957).) You administer the same survey to each associate.
Create the Design
Construct a table that lists the items or profiles for your choice sets. In this example, your table of items, Candy Profiles.jmp, is already constructed.
1. Select Help > Sample Data Library and open Design Experiment/Candy Profiles.jmp.
The table lists the seven candy types of interest.
2. Select DOE > Consumer Studies > MaxDiff Design.
3. From the Select Columns list, select Candy and click X, Factor.
4. Click OK.
5. In the Design Options outline, do the following:
‒ Set the Number of Profiles per Choice Set to 4.
‒ Set the Number of Choice Sets to 7.
Note: Setting the Random Seed in step 6 reproduces the exact results shown in this example. In constructing a design on your own, this step is not necessary.
6. (Optional) Click the MaxDiff Study red triangle and select Set Random Seed. Type 12345 and click OK.
7. Click Make Design.
The Design outline shows 7 choice sets, each consisting of 4 candy types.
8. Click Make Table.
Figure 19.2 Design for Candy Preference Survey
The design table contains a Choice column for recording preferences. For each choice set, record a 1 for the most preferred candy, a -1 for the least preferred, and a 0 for the other two candies.
Analyze the Study Results
You conduct the study and record your data in Candy Survey.jmp.
1. Select Help > Sample Data Library and open Design Experiment/Candy Survey.jmp.
The table shows the results of presenting the survey to each of five respondents, listed in the Subject column.
2. Select Analyze > Consumer Research > MaxDiff.
3. Click Select Data Table, select Candy Survey, and click OK.
4. Assign roles to columns as follows:
‒ Select Choice and click Response Indicator.
‒ Select Subject and click Subject ID.
‒ Select Choice Set and click Choice Set ID.
‒ Select Candy and click Add in the Construct Profile Effects panel.
Figure 19.3 Completed MaxDiff Analysis Launch Window
Because you designated the Best choice as 1 and the Worst choice as -1, you make no change to the Best and Worst choice indicators at the bottom left of the launch window.
5. Click Run Model.
Figure 19.4 MaxDiff Report
The report indicates that Candy is significant. The three candy types with the highest utilities are Plain M&Ms, Reese’s Cups, and Peanut M&Ms.
6. Click the MaxDiff Model red triangle and select All Levels Comparison Report.
Figure 19.5 All Levels Comparison Report for Candy Types
The comparison report indicates which pairs of candy types differ significantly in terms of utility. The third entry in each cell is the p-value for the difference defined by the row item’s utility minus the column item’s utility. The intensity of the color for the p-value indicates how significant a difference is. The shading, blue or red, indicates whether the difference (row - column) is negative or positive. The p-values are not adjusted to control the multiple comparison error rate and should be used only as a guide. For details about the All Level Comparisons Report, see the MaxDiff chapter in the Consumer Research book.
MaxDiff Design Launch Window
To use the MaxDiff Design platform, you need a starting data table. Your starting data table must contain a column of character data that lists the items to be presented to respondents for rating.
With your starting data table active in JMP, select DOE > Consumer Studies > MaxDiff Design. If you have no active data tables, you are prompted to navigate to your starting data table.
Figure 19.6 MaxDiff Launch Window using Candies.jmp
X, Factor
The character column that contains the items that respondents will rate.
MaxDiff Window
The MaxDiff Study window updates as you work through the design steps. The outlines, separated by buttons that update the outlines, follow the flow in Figure 19.7
Figure 19.7 MaxDiff Design Flow
The MaxDiff Study window opens showing the Design Options outline. Once you click Make Design, the Design outline appears. To construct the design table, click Make Table.
Design Options Outline
Specify the following:
Number of Profiles per Choice Set
The number of items to be included in each choice set.
Number of Choice Sets
The total number of sets of items to be presented to and rated by respondents.
Design Outline
The Design outline identifies the choice sets using consecutive positive integers. The items that comprise each choice set are listed. In the Design outline, you can review the design settings.
Make Table
The Make Table button creates the design table. The design table consists of four columns:
Subject
Initially populated by ones. Replace with appropriate identifiers for respondents.
Tip: To easily add respondent identifiers, see the section Fill Columns with Sequential Data in the Enter and Edit chapter of the Using JMP book. Use Columns >Recode to change the identifiers to names.
Choice Set
A designator for each choice set.
Factor
The levels of the factor that you specified. These are the items in the choice set.
Choice
A column where you can enter results. Use a numeric value to indicate the best choice, the worst choice, and choices in between. The values 1, -1, and 0 are typically used and are required for analysis by the MaxDiff analysis platform (located at Analyze > Consumer Research > MaxDiff).
MaxDiff Options
Set Random Seed
Sets the random seed that JMP uses to control certain actions that have a random component. These actions include:
‒ simulating responses using the Simulate Responses option
‒ randomizing Run Order for design construction
‒ selecting a starting design
To reproduce a MaxDiff design or simulated responses, enter the random seed and Number of Starts that generated them. Do this step before clicking Make Design.
Simulate Responses
Select this option to simulate response values. When you click Make Table, a Simulate Choice window opens along with the design table, and Probability and Choice Simulated columns are added to the design table. The Choice Simulated formula column contains random responses and the Probability formula column contains their probabilities.
To change the model used in simulating the responses, enter values in the Marginal Utility column in the Simulate Choice window for all factor levels but the last. Because the marginal utilities must sum to zero, you are not permitted to edit the Marginal Utility for the last level. Once you have specified the remaining Marginal Utility values, the last level of the factor is adjusted accordingly.
Number of Starts
The number of random starts used in constructing the design. This value is set to 10 by default.
Advanced Options
None applicable.
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