Why this is mistaken
A supplied curve is a model input unless its empirical provenance says more. Even an estimated association between price and quantity can mix movements and shifts because market price and quantity are jointly determined.
Repair the interpretation by labeling the function as stipulated, calibrated, or estimated; state units and domain; identify held-constant determinants; and describe the design needed for a causal claim. Test whether conclusions survive a reasonable alternative functional form and stop extrapolation where quantities or institutions become implausible.
Reproducible arithmetic verifies consequences of the curve. It does not verify that the curve describes the market or identifies a causal mechanism.
Where to watch
When this mistake may appear
- A worksheet supplies a linear equation.
- Observed price and sales change together.
- An intercept lies outside the relevant domain.
Check your work
Your work may contain this mistake if:
- Calls every point on the line an observed transaction.
- Reports the slope as a causal estimate by definition.
- Extrapolates through negative quantities or implausible prices without a domain warning.