The Core Problem
Index numbers assume readers understand:
- What the base year is
- That bases change over time
- How to calculate percentage change correctly
Most people don’t. The system works for economists talking to economists. It’s fragile for public communication.
The Two Traps
Trap 1: Wrong percentage calculation
Index goes from 125 → 132.5.
- Wrong: “That’s 7.5%”
- Right: (132.5 - 125) / 125 = 6%
Trap 2: Comparing across different base years
“CPI was 120 in 2010, now it’s 108… prices fell?”
Maybe. Or maybe the base year changed. Without checking, you can’t know.
Why It Usually Doesn’t Cause Chaos
Most reporting uses percentage change (“inflation was 3.4%”), not raw index levels. Percentage change is base-agnostic—same calculation regardless of base year.
When It Breaks Down
| Context | Risk |
|---|---|
| Official statistics (StatCan) | Low—base year documented |
| News headlines | Medium—base year often omitted |
| Comparing old vs. new articles | High—bases may differ silently |
The Takeaway
When you see a raw index number, ask: base year?
When comparing index numbers across time from different sources, verify they’re using the same base before drawing conclusions.
North: Where this comes from
- ECON-1221 Chapter 2 - Notes from the Textbook (Introduction to base numbers)
- Index Numbers (the underlying concept being misunderstood)
- Consumer Price Index (CPI) (most common public-facing index)
East: What’s the opposite?
- Percentage Change (what people should focus on—base-agnostic)
South: Where this leads
- Misleading Statistics in Media (consequence of low statistical literacy)
- How to Read Economic Data (practical application)
West: What’s similar?
- Correlation vs Causation (another common statistical misunderstanding)
- Nominal vs Real Values (related confusion about what numbers actually mean)