What Is Percentage Difference?
Percentage difference measures how far apart two values are relative to their average. Unlike percentage change (which compares a new value to an old value), percentage difference treats both values equally — there is no "starting" or "ending" value. It is used when you want to compare two measurements, test results, or data points without implying that one caused the other.
The Percentage Difference Formula
Or simplified:
Percentage Difference = (|Difference| ÷ Average) × 100
Step-by-Step Example
Two labs measured the density of a substance: Lab A got 3.42 g/cm³, Lab B got 3.51 g/cm³:
- Find the absolute difference: |3.42 - 3.51| = 0.09
- Find the average: (3.42 + 3.51) ÷ 2 = 3.465
- Divide difference by average: 0.09 ÷ 3.465 = 0.02597
- Convert to percentage: 0.02597 × 100 = 2.6%
Percentage Difference vs Percentage Change
Percentage difference and percentage change are NOT the same calculation. Use percentage change when there is a clear "before and after." Use percentage difference when comparing two independent values.
| Feature | Percentage Difference | Percentage Change |
|---|---|---|
| Formula base | Average of both values | Original (old) value |
| Direction | Always positive | Can be positive or negative |
| Order matters? | No | Yes |
| Use case | Comparing two measurements | Tracking change over time |
| Example | Comparing two test scores | This year vs last year |
Calculation Comparison
Values: 50 and 60
| Method | Calculation | Result |
|---|---|---|
| % Difference | |50-60| ÷ ((50+60)÷2) × 100 | 18.2% |
| % Change (50→60) | (60-50) ÷ 50 × 100 | 20% increase |
| % Change (60→50) | (50-60) ÷ 60 × 100 | 16.7% decrease |
Notice how percentage difference gives one answer (18.2%), while percentage change gives different answers depending on direction.
When to Use Percentage Difference
- Comparing measurements from two instruments: "How different are these two readings?"
- Comparing prices at two stores: "How much do these prices differ?"
- Comparing test results from two groups: "How far apart are these scores?"
- Quality control: Checking consistency between batches or samples.
- Scientific experiments: Comparing results from repeated trials.
Practice Examples
| Value A | Value B | Difference | Average | % Difference |
|---|---|---|---|---|
| 100 | 110 | 10 | 105 | 9.5% |
| 250 | 300 | 50 | 275 | 18.2% |
| 45 | 42 | 3 | 43.5 | 6.9% |
| 1,000 | 1,050 | 50 | 1,025 | 4.9% |
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