A regression coefficient measures how the modeled outcome changes when one predictor increases by one unit while the other included predictors remain fixed. Linear regression estimates the coefficients that best fit observed data, making the result both practical and interpretable for continuous variables. section 6.0
Airline price and demand
Suppose demand is modeled from ticket price and whether a flight occurs on a holiday. The price coefficient describes the change in booked seats associated with a one-unit price increase while holiday status stays unchanged. That interpretation is descriptive unless the model also addresses confounding: an airline might lower prices during economic crises and raise them in stronger periods, when demand is affected by other causes. section 6.0
The same coefficient can support different questions depending on the causal structure. A confounder can make the unadjusted coefficient misleading; including the confounder in Y ~ X + Z can estimate X's coefficient when the adjustment is appropriate. A mediator separates the direct effect from the total effect, whereas conditioning on a collider can create misleading correlation. Regression coefficients therefore require more than a good fit: interpret the variables and paths before treating them as causal effects. section 6.1.2 section 6.2.2
Use a regression coefficient by asking:
- What outcome and predictor does it connect, and what units express a one-unit change? section 6.0
- Which variables should be held fixed because they are confounders, and which should remain unconditioned because they are mediators or colliders? section 6.1.2
- Does the coefficient represent a direct effect, a total effect, or merely an association? section 6.1.2




