Margin of Error

1
Margin of Error

• or
• Horseshoes and Hand grenades

2
Review

• Population
• entire group about which we want information
• Parameter
• numerical characteristic of the population
• Sample
• a collection of individuals from the population
• Statistic
• numerical characteristic of the sample

3
Types of Parameters

• A parameter is a numeric characteristic of the
  population
• Types of Parameters

4
Types of Statistics

• A statistic is a numeric characteristic of a
  sample
• Types of Statistics

5
M.O.E.

• Margin of error is a way to convey the
  uncertainty in our sample statistic

6
Example

• As Democrats and Republicans begin their public
  relations battle over a "patient's bill of
  rights," a new CNN/USA Today/Gallup poll shows
  that more than three in four Americans say such a
  bill is either "extremely" or "very" important to
  them. Both parties in Congress support some form
  of a patient's bill of rights, and the Senate is
  ready to debate the two different measures -- one
  backed by President Bush and the Republican
  leaders in the Senate, the other backed by Senate
  Democratic leaders. The poll, conducted June
  8-10, shows that 77 of Americans say it is
  "extremely" or "very" important, that Congress
  and the president address the issue of a
  patient's bill of rights this year. Results are
  based on telephone interviews with 1,011national
  adults, aged 18, and conducted June 8-10, 2001.
  For results based on the total sample of national
  adults, one can say with 95 confidence that the
  margin of sampling error is /- 3 percentage
  points. In addition to sampling error, question
  wording and practical difficulties in conducting
  surveys can introduce error or bias into the
  findings of public opinion polls.

7
Patient Rights

• Population
• Parameter
• Sample
• Statistic

8
Margin of Error

• Margin of error 3
• A quick method for calculating the margin of
  error for a 95 confidence statement
• M.O.E.

9
Confidence

• Confidence statements

10
Confidence

• The size of the population does not influence the
  margin of error.
• A larger sample size will result in a smaller
  margin of error.

11
Big Drawback

• Margin of error

12
Notes

• Notes
• The conclusion of a confidence statement always
  applies to the population not to the sample.
• Our conclusion about the population is never
  certain.
• A sample survey can use a confidence level other
  than 95.
• a. 90, 99...
• b. Higher confidence gt bigger margin of error
• The size of the population does not influence the
  margin of error, sample size matters.
• A larger sample size will result in a smaller
  margin of error.
• a. To cut the margin of error in half, we must
  use a sample four times as large.