Controlled experiments in economics: On April 7, 2000, President Clinton signed into law a bill passed by both Houses of the U.S. Congress that lifted earnings limitations on Social Security recipients. Until then, recipients between the ages of 65 and 69 who earned more than $17,000 a year would lose $1 worth of Social Security benefit for every $3 of income earned in excess of $17,000. How would you devise a study to assess the impact of this change in the law?