John Wiley & Sons TI-83 User Manual download pdf (Page 53)

Example: Legal Advice

According to a 2002 survey by FundLaw, 20% of Americans needed legal advice during the past

year to resolve such thorny issues as trusts and landlord disputes. Suppose a recent sample of

1000 adult Americans showed that 20% of them needed legal advice in the past year to resolve

such family-related issues. Find a 99% confidence interval for the percentage of American

adults who needed such legal advice.

In our sample of 1000 American adults, there were 20% or 200 successes and 800 failures. We

can use 1-PropZInt with x = 200, n = 1000, and our C-Level set at 0.99.

Press the STAT key.

Press the ► key twice to get to TESTS.

Press the ALPHA key and then the MATH key to get

to the letter A.

Type in 200 for x.

Type in 1000 for n.

Type in .99 for C-Level.

Highlight Calculate and press the ENTER key.

The 1-PropZInt output shows the 99% confidence

interval, the sample proportion, and the sample size.

The 99% confidence interval is from .167 to .233 which

means that we believe with 99% confidence that the

true population proportion is between 16.7% and 23.3%

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John Wiley & Sons TI-83 User Manual Page 53