PROJECTQuantitative Data Analysis

• Project Home
• Inference
• Impact Analysis
• Bias
• Experiments
• Paired Testing
• Quasi-experimental Methods
• Difference-in-Difference and Panel Methods
• Instrumental Variables
• Propensity Score Matching
• Regression Discontinuity
• Regression Techniques
• Generalized Linear Model
• Linear Regression
• Logit and Probit Regression
• Segregation Measures
• Inequality Measures
• Decomposition Methods
• Descriptive Data Analysis
• Microsimulation
• The Dynamic Simulation of Income Model DYNASIM
• The Health Insurance Policy Simulation Model HIPSM
• The Model of Income in the Near Term (MINT)
• The Tax Policy Center Microsimulation Model
• The Transfer Income Model TRIM
• Performance Measurement and Management

• For a binary outcome (yes or no; success or failure), we assign y = 0 for one outcome and y = 1 for the other, and the logit or logistic regression models E(y|X) as a nonlinear function of Xb, 1/(1+exp(-Xb)). For a fractional outcome that lies between 0 and 1, we can again assume E(y|X) = 1/(1+exp(-Xb)), and both models can be estimated using generalized linear models.

Estimates from a logit or fractional logit model are often expressed in odds ratios or log odds, a common measure of effect size for proportions. Given a proportion, fraction, or probability p, the corresponding odds are p/(1-p), and an odds ratio for two fractions p and q is p/(1-p) divided by q/(1-q). Odds ratios are multiplied together, but log odds can be added for the same effect.

Interpretation of logit estimates depends on whether coefficients are reported as effects on log odds or on odds ratios. Thus, a logit coefficient on X of 0.5 shows an increase in a fraction successful (y = 1) when X increases by one unit, and a coefficient of 0 shows no impact. On the odds ratio scale, the same coefficients would be 1.6487 and 1, so the no-impact comparison point is always 1 on the odds scale.

For a binary outcome, we assign y = 0 for one outcome and y = 1 for the other, and the probit regression models E(y|X) as cumulative normal distribution of Xb. In these regressions, coefficients have no natural interpretation and scale is arbitrary; only ratios of different coefficients are identified. Often, we seek to convert logit or probit regression results back to the probability or fraction scale, which requires computing marginal effects.

Research Methods Data analysis Quantitative data analysis Research methods and data analytics