Potential Problems with Sampling Practice Test

A town wants to estimate the proportion of all registered voters in the town who support a proposed bond measure. The town clerk takes the voter registration list, sorts it alphabetically by last name, and selects every 20th name starting with the first name on the list. The clerk assumes this produces a representative sample. Which issue most threatens the validity of this sampling method?

A city wants to estimate the mean number of minutes commuters spend on public transit each weekday for all city residents who commute. Surveyors stand at one downtown subway station from 7:00–8:00 a.m. on a Tuesday and ask every 5th rider entering the station how long their commute is. The city reports the result as representing all commuters in the city. Which issue most threatens the validity of this sampling method?