Name the major threat to internal validity.
The Solow growth model predicts that the GDP per capita of countries with similar technology and savings behaviour will converge over time. To test this theory a researcher examines the growth of G7 countries, fails to reject convergence, and concludes that the Solow model's predictions are correct.
To test the deterrent act of capital punishment on violent crime in the U.S., a researcher regresses state violent crime rates on whether the state has capital punishment and all other relevant variables. The researcher concludes that capital punishment increases a state's violent crime rate.