Will cash bonuses speed the return to work of unemployed people?

Will cash bonuses speed the return to work of  unemployed people? A staate department of labor notes that last year 68% of people who filed claims

under a new job in 15 weeks. This year the state offers $500 to people who file claims if they find a job in 15 weeks. The percent rises 77%. Wat flaw in he desing makesi

impossible to say whether the bonus really caused the increase?