What sources of error might you encounter if you want to know
(a) About the dating habits of college men, so you go to a dorm meeting and ask students how many dates they have had in the last year;
(b) How often people attend religious services, so you stand outside a particular church on Sunday and ask entering individuals how often they attend;
(c) How often people eat at McDonald’s, so you stand outside a particular McDonald’s and ask entering customers how often they eat at McDonald’s?