Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

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Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Warm Up

A principal wants to know how often students at her school visit museums. Describe how she could use each method to select a sample of students.

1. random sample

2. convenience sample

3. self-selected sample

Randomly choose 50 students from a list of all students.

Survey 30 students in one classroom.

Place survey forms in the school cafeteria.

Possible answers:

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

SDAP2.4 Identify data that represent sampling errors and explain why the sample (and the display) might be biased.Also covered: SDAP2.3, SDAP2.5

California Standards

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Vocabulary

biased samplebiased question

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

A biased sample is a sample that does not fairly represent the population.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Determine whether each sample may be biased. Explain.

Additional Example 1: Identifying Potentially Biased Samples

A. An employee of a phone company randomly chooses 80 customers to ask whether they are happy with their service.

The sample is not biased. It is a random sample.

B. The first 25 students to arrive at school are surveyed to find out how they get to school.

The sample is biased. It is likely that the first 25 students arrived by the same method, such as the bus.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Determine whether each sample may be biased. Explain.

Check It Out! Example 1

A. A sales manager of a large department store randomly chooses 40 previous customers to ask whether they were satisfied with the customer service they received.The sample is not biased. It is a random sample.

B. The last 50 people to leave a theater production are surveyed to find out how the town enjoyed the performance.

The sample is biased. It is likely that those who stay enjoyed the production more than those who were quick to leave.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

The results of a survey may also be affected by biased questions. A biased question is one that leads people to give a certain answer.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Determine whether each survey may be biased. Explain.

Additional Example 2: Identifying Potentially Biased Questions

A. Do you prefer broccoli or corn?

The question is not biased. It does not lead people to choose one item over the other.

B. Would you rather drive this luxurious and roomy sedan or this tiny compact car?

The question is biased. People may be more likely to choose a car that is described as luxurious and roomy than one that is described as tiny and compact.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Determine whether each survey may be biased. Explain.

Check It Out! Example 2

B. Do you prefer Sunday or Friday?

The question is not biased. It does not lead people to choose one day over the other.

A. Would you rather have a new wide screen or an old small screen television?

The question is biased. People may be more likely to choose a television that is described as new with a wide screen than one that is described as old with a small screen.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

To decide whether a claim based on a survey is valid, check whether the sample or the survey question is biased. In addition, make sure that the sample was taken from the correct population.

Holt CA Course 1

7-8 Identifying Sampling Errors and BiasAdditional Example 3: Evaluating Survey Claims

Mari conducts a survey to find out which animals at the zoo are the most popular among visitors. Determine whether Mari’s claim is valid. Explain.

Sample: 30 visitors outside the monkey house

Claim: Monkeys are the most popular animal.

Favorite Zoo Animals

Animal Responses

Monkeys 10

Bears 8

Lions 6

Question: Which animals at the zoo are your favorite?

Results:

The claim may not be valid. The sample is biased because visitors standing outside the monkey house may be more interested in monkeys.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Check It Out! Example 3

Jed conducts a survey to find out which animals at the zoo are the most popular among visitors. Determine whether Jed’s claim is valid. Explain.

Sample: 50 visitors selected at random throughout the day

Claim: Lions are the most popular animal.

Favorite Zoo Animals

Animal Responses

Monkeys 12

Bears 18

Lions 20

Question: Which animals at the zoo are your favorite?

Results:

The claim is valid. The sample is random.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

Lesson Quiz: Part I

1. Determine whether the sample may be biased. Explain. A dentist chooses 30 patients to survey by selecting names at random from his database.

2. Determine whether the survey question may be biased. Explain. Given its enormous cost, do you support a space mission to the moon?

The sample is not biased. It is a random sample.

Biased; the words enormous cost lead people to answer no.

Holt CA Course 1

7-8 Identifying Sampling Errors and Bias

3. Vietta conducts a survey to find out whether students would like a salad bar in the cafeteria. Determine whether Vietta’s claim is valid. Explain.

Sample: 50 names chosen at random from the school’s enrollment list

Question: Many students have said they would like a salad bar; do you agree?

Results:

Claim: Most students want a salad bar added to the cafeteria.

Lesson Quiz: Part II

No; the question is biased. It leads students to answer yes.

Survey: Salad Bar

Yes 76%

No 20%

Don’t know 4%