| A video rental chain wants to conduct a survey to determine what kind of movie teenagers like to watch. Comedy? Science Fiction? Action? How should the manager conduct the survey? |
Because the manager cannot possibly survey every teenager, a smaller group, a sample, must be chosen. When choosing a sample, it’s important that it is representative of the population.
Suppose the manager decided to survey a group of people at the local grocery store. Would this group represent the entire population? Not likely. Teenagers probably aren’t going to be hanging out at the grocery store. Therefore, the sample isn’t likely to be made up of a bunch of teenagers (which is the population).
What about a group of teenagers standing in line for the evening comedy show? This sample would be made up of teenagers, but they’re likely to be biased. In a biased sample, a certain outcome is favored or encouraged. Because these teenagers are standing in line for a comedy show, they probably prefer comedy movies.
There are two ways to select a biased sample: a convenience sample and a voluntary response sample. In a convenience sample, individuals that are most convenient are selected to be part of the sample. A voluntary reponse sample gives individuals the option to be a part of the sample.
For the most valid results, an unbiased sample should be selected. The most common unbiased sample is a simple random sample. In a simple random sample, each person in the population is equally likely to be selected.
| Biased Samples | |
| CONVENIENCE SAMPLE | The video rental manager surveys the employees of the store and all of his friends. |
| VOLUNTARY RESPONSE SAMPLE | The video rental manager mails a survey to everyone in town. They are asked to complete the survey and mail it back to him. |
| Unbiased Sample | |
| SIMPLE RANDOM SAMPLE | The video rental manager survey’s every 10th customer to enter the store. |
The video rental manager decided to mail the survey to everyone in town. The results indicated that teenagers prefer comedies. Are these results valid? Predictions based on the results of biased samples are not always valid. Because the manager chose a biased, voluntary response sample, these results are not valid. What might have been a better choice for a sample?
Determine whether each sample method is unbiased or biased. If the method is unbiased, use it to make a prediction. If the method is biased, explain why.
1. A cable company representative randomly calls 250 households to determine how many customers subscribe to the movie channels. Of the 250 households contacted, 22% do not subscribe to movie channels. If there are 5,240 customers, how many do not subscribe to movie channels.
2. Pete’s Twisted Pretzels just received a new shipment of pretzel sticks. The manager must make sure that the shipment has not been damaged. There are 100 boxes, each containing 50 bags of pretzels. The manager decides to examine only the nearest 5 boxes. He finds only 2 damaged packages. What can he conclude about the entire shipment?
3. The pizza restaurant asked its customers to visit a Web site to indicate their favorite pizza topping. 40% of the visitors said pepperoni, 30% said sausage, 25% said cheese and 5% said mushroom. What can the pizza restaurant conclude?
