A fixed or constant sum rating scale is a type of measurement scale where respondents allocate a fixed number of points, percentages, or other units across a set of attributes or objects to reflect their relative importance, preference, or value.
How Fixed Sum Scales Work
In essence, participants are given a "budget" of points and must distribute them among the items being rated. The total sum of points must equal the predetermined fixed amount. This constraint forces respondents to make trade-offs and prioritize, offering richer data than simple rating scales where items can be rated independently.
Key Characteristics
- Fixed Total: Respondents must distribute a predefined number of points (e.g., 100 points, 10 points) or a percentage (e.g., 100%).
- Relative Assessment: It encourages respondents to think about the relative importance or value of each item being assessed compared to others.
- Forced Trade-offs: Because the sum is fixed, allocating more points to one item necessarily requires reducing the points allocated to another.
- Provides Ranking Data: The points assigned provide a ranking and relative weighting of the items being evaluated.
Example
Imagine you want to understand what factors are most important to customers when choosing a restaurant. You could use a fixed sum scale, asking respondents to allocate 100 points among the following attributes:
| Attribute | Points Allocated |
|---|---|
| Food Quality | 40 |
| Price | 20 |
| Atmosphere | 15 |
| Service | 20 |
| Location | 5 |
| Total | 100 |
In this example, the respondent believes food quality is the most important factor, followed by price and service. Location is the least important.
Advantages
- Discriminatory Power: Provides more nuanced data compared to simpler rating scales.
- Forces Prioritization: Helps uncover true preferences by requiring respondents to make trade-offs.
- Ratio Scale Data: Generates ratio-scale data, allowing for more sophisticated statistical analysis.
Disadvantages
- Cognitive Load: Can be more demanding for respondents, requiring careful consideration of allocations.
- Potential for Errors: Respondents might struggle to ensure the total sum equals the fixed amount.
- Limited Number of Items: Can be difficult to use effectively with a large number of items to rate.
Fixed or constant sum scales are particularly useful in market research, product development, and decision-making processes where understanding relative importance and priorities is critical.
