Airline boarding passes are marked with red stars at random to decide which passengers should have their carry-on luggage inspected.

To determine the most common injury cared for in an emergency room, a reporter goes to the same hospital every afternoon for one month during the summer and observes people entering the emergency room. She concludes that second degree sunburn is the most common injury.

To evaluate customer satisfaction, a grocery store gives double coupons to anyone who completes a survey as they enter the store. The store manager determines that customers are very satisfied with their shopping experience in his store.

Carlos wants to know the favorite sport of people at his school so he asks his baseball team.

A homeowner divides their backyard into 5 sections, and randomly removes soil from sections.

A public library asked every 10th person who entered to name the type of book they were getting.

A company makes DVDs and randomly selects 25 from 1500 to test them to see if they work.

To see who has a computer at home the front office chooses 10 students randomly per grade to ask.

A senator wants to know how she is doing in her position. She sends a survey to 1000 registered voters in her party.

To get valid results, a sample must be...