Samples, Surveys, and Identifying Bias

Algebra AUnit 3, Lesson 9

What is Sampling?

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Definition:

The population is the entire group of individuals about which we want information.

A sample is the part of the population from which we actually collect information. We use information from a sample to draw conclusions about the entire population.

PopulationPopulation

SampleSample

Collect data from a representative Sample...

Make an Inference about the Population.

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Example:

The cafeteria staff wants to know how many students in the school enjoy pizza for lunch. They send Mrs. King to ask a group of 40 students if they like the cafeteria pizza.

What is the population? What is the sample?

Three methods of sampling:

1. Random:• Survey a population at random.

2. Systematic:• Select a random number n and survey every

nth person.

3. Stratified:• Separate the population into smaller groups

and survey a random sample in each group.

Example:

Of the 40 students surveyed, 28 state that they like the school’s pizza. If the school has 500 students that buy lunch, how many students should they expect to buy pizza?

Definition:Choosing individuals who are easiest to reach results in a convenience sample.

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Bad Samples:

Definition:A voluntary response sample consists of people who choose themselves by responding to a general appeal.

Bias…

…in a survey question.

“Do you prefer exciting action movies or studious documentaries?”

Bias…

…in survey results.

A sports magazine sends out a survey to its subscribers asking them to identify their favorite sport. As incentive, anyone who responds will receive a free box of golf tees.