In exercises that call for two-sample procedures, you may use either of the two approximations for the degrees of freedom that we have discussed: the value given by your software or the smaller of and . Be sure to state clearly which approximation you have used.
7.58 Dust exposure at work.
Exposure to dust at work can lead to lung disease later in life. One study measured the workplace exposure of tunnel construction workers.29 Part of the study compared 115 drill and blast workers with 220 outdoor concrete workers. Total dust exposure was measured in milligram years per cubic meter (). The mean exposure for the drill and blast workers was with a standard deviation of . For the outdoor concrete workers, the corresponding values were 6.5 and , respectively.