Short answers, pulled from the story.
Arthur Melvin Okun presented his paper titled Potential GNP: Its Measurement and Significance in the autumn of 1962 before the Business and Economics Statistics Section of the American Statistical Association. This presentation marked the first formal link between unemployment rates and national production losses.
A standard estimate suggests that a one-point increase in cyclical unemployment creates a two percentage point drop in real GDP relative to potential. This ratio became known as Okun's law shortly after Okun died in 1980.
The gap version calculates how much actual GDP falls short of potential GDP when unemployment rises above its natural rate. The difference version offers a more practical alternative by tracking quarterly changes in unemployment against changes in actual output over the same period without needing to guess at potential GDP figures.
Different coefficients arise because economies do not react uniformly to shocks or policy changes due to unique labor laws or industrial structures. Structural factors such as jobless growth, reduced hours worked, and workers dropping out of the labor force also alter the observed association.
Forecasters use Okun's law primarily for short-run trend analysis rather than long-term predictions because it serves as an invaluable tool for predicting trends within specific quarters or years. Historical data from recessions occurring in the 1970s, 1990s, and 2000s show that the theory holds higher accuracy for immediate forecasting needs.