Warm UpThe Ohio Department of Health wishes to estimate the percentage of Ohio State students who smoke cigarettes at each of Ohio State's campuses. 100 names are picked at random from the list of 50,000 registered students at the main campus and another 100 names are picked from the 1,700 registered students at the Mansfield campus. The Department finds that the percentage of students that smoke at the main campus has a margin of error of 8%. The percent that smoke at both of the two campuses is found to be about the same. Would the margin of error for the Mansfield campus be; clearly greater than 8%, clearly less than 8%, or close to 8%? Pick one and explain briefly.

Sampling Error

Sampling errors are errors caused by the act of taking a sample. They cause sample results to be different from the results of a census.

Think—Pair—ShareWhat are some sampling errors you might expect to misrepresent the results?

Sampling Errors

• Bad sampling methods (voluntary response)

• Size of sample

• Not beginning the sampling process with the entire population accounted for.

• Leaving out groups of the population.

Sampling Error

Under-coveragePopulation that is actually sampled is not as broad as the population that the researcher desires to sample (the target population)Eight people asked about an entire city’s garbage collection

Adapted from Exploring Surveys and Information from Samples Application 20

Book Example

Turn to pg. 238 in your textbook

Read Example 5.13 at the bottom of the page.

Discuss with a partner what you have learned.

Can you come up with any other types of under-coverage?

Non-Sampling Errors

The more common errors happen when someone picks up (or doesn’t pick up) the phone.

These errors are called non-sampling errors because the error is not related to the sampling process.

Non-Sampling Errors

Processing ErrorsThese are mistakes in mechanical tasks such as doing arithmetic or entering responses into a computer.

Non-Sampling Errors

Response ErrorOccurs when someone gives an incorrect response.

Let’s look at 4 examples…

Response Error Example

Untruthful answersPeople give untruthful answers for several reasons, such as: sensitive questions, socially acceptable answers, telling the interviewer what he or she wants to hear. What is a current sensitive issue?

The fix: secret ballots, anonymous surveys, "sensitive question" techniquesAdapted from Exploring Surveys and Information from Samples Application 20

Response Error Example

Ignorant people People who don't want to appear like they know nothing about the subject. Example: In a study educators were asked how they would rank Princeton's undergraduate business program. In every case, it was rated among the top 10 departments in the country, even though Princeton doesn't offer an undergraduate business major.

Adapted from Exploring Surveys and Information from Samples Application 20

Response Error Example

Don't know, haven't decidedSurveyors may interpret these responses to flatter the response they want.Example:Do you regularly worry about money?

Yes No I Don’t Know

Adapted from Exploring Surveys and Information from Samples Application 20

Response Error Example

People who don't remember the actual answer. Example: Students were asked to report their grade point averages. Researchers then determined the actual GPA's. Over 17% of the students reported a GPA that was .4 or more above their actual average, and about 2% reported a GPA more than .4 below their actual GPA. (more inflated their GPA's!)

Adapted from Exploring Surveys and Information from Samples Application 20

Non-Sampling Errors

NonresponseMissing data, people who hang up, or refuse.Nonresponse is the most serious problem facing sample surveys.Example: Fred sent out 2000 surveys to Boise households. Only 481 surveys were returned. What was the response rate? Adapted from Exploring Surveys and Information from Samples Application 20

Non-Sampling Errors

TimingWhen the sample is taken.In January the National Football League reported a poll that revealed football as the nation's favorite.Why do you think that was?

Adapted from Exploring Surveys and Information from Samples Application 20

Non-Sampling Errors

Statement of questionsSubtle differences in phrasing make large differences in the results.10% said they would support cutting programs involving "aid to the needy" 39% said they would support cutting programs to "public welfare programs"

Adapted from Exploring Surveys and Information from Samples Application 20

Margin of ErrorThe announced margin of error that you see in confidence statements accounts only for sample errors. Under-coverage, nonresponse, and other errors can cause large bias that is not covered by the margin of error.

So what do the results really say? Can we trust any results?

Book Example

To answer, “What can we trust?” turn to page 249 in your textbooks.

Read this page. Get familiar with the questions to ask before accepting a poll’s results.

Summary1. Under-coverage2. Processing Errors3. Response Errors

• Don’t Know/Haven’t Decided• Untruthful• Ignorant• Don’t remember actual number

4. Nonresponse5. Timing6. Statement of question