. A 1997 study of armed robbers in St. Louis, MO recruited participants through a "snowball" sample using word-of-mouth social networks, not through involvement in the criminal justice system*. As such, not all participants had ever been arrested for armed robbery. One of the questions the researchers asked in their survey was if participants had ever been arrested for armed robbery during their criminal careers. A frequency table of the results is presented below in Table 1:

Table 1. Have you ever been arrested for armed robbery?

Never Arrested

28

Arrested, not convicted

23

Convicted of Armed Robbery

35

Total

86

a. What proportion of participants had never been arrested for armed robbery?

b. What notation would you use to describe the quantity you calculated in part A?

c. What is the percentage of participants who had been convicted of armed robbery?

d. Based on the information provided, do you think this was an observational study or an experiment? Why?