| Y, y | Outcome variable | Economic response being explained |
| X, x | Regressor or full design matrix | Observed explanatory variation |
| β, β̂ | Population parameter, sample estimate | Target versus computed result |
| u, û | Structural disturbance, fitted residual | Unobservable cause versus fitted discrepancy |
| n, T | Observations, time periods | Sample dimensions |
| E[·], Var(·) | Expectation, variance | Population probability operations |
| RSS, TSS, ESS | Residual, total, explained sums of squares | In-sample variance decomposition |
| SE, HC3 | Standard error, leverage-adjusted robust SE | Sampling uncertainty |
| D, Z | Treatment, instrument | Endogenous variable and external variation |
| αi, λt | Entity and time effects | Unobserved constant and common temporal components |
| ρ, φ | Correlation, autoregressive coefficient | Dependence measures |
| λ | Penalty parameter | Regularisation strength |
| ATE, ATT, LATE | Treatment-effect targets | Population, treated, complier averages |
