1. How do I study different sampling methods for collecting data?
Lesson 11.4

2. Population – who you want to know about Sample – who you actually study You want the sample to accurately portray the population.

3. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Types of Sampling
Self-selected sampling:
The people being surveyed choose whether or not to participate in the survey.
Systematic sampling:
Involves a random start and then proceeds with the selection of every kth element from then onwards.
Convenience sampling:
A sample population is selected because it is readily available and convenient for the person conducting the survey.
Random sampling:
A sampling where each member of the population has an equal and known chance of being selected.

4. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Example 1
Classify samples
A student wants to survey students at his school about the quality of the school’s assemblies. Identify the type of sample described as a self-selected sample, a systematic sample, a convenience sample, or a random sample.
The student surveys every 8th student that enters the assembly.
Solution
The student uses a rule to select students, so the sample is a ____________ sample.

5. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Example 1
Classify samples
A student wants to survey students at his school about the quality of the school’s lunches. Identify the type of sample described as a self-selected sample, a systematic sample, a convenience sample, or a random sample.
From a random lottery, the student chooses 125 students to survey.
Solution
The student chooses from a random name lottery, so the sample is a __________ sample.

6. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Example 2
Identify the type of sample described.
A local mayor wants to survey local area registered voters. She mails surveys to the individuals that are on her mailing list and uses the surveys that are returned.
Convenience and….

7. Select and Draw Conclusions from Samples
7.5
Select and Draw Conclusions from Samples
Example 3
Identify biased solutions
Tell whether each sample in Example is biased or unbiased. Explain your reasoning. Does the sample represent the intended population fairly?
Ann Lander’s asks readers to answer a survey about whether they are happy with their marriage.
You stand outside the cafeteria after lunch and ask every 3rd student how many times they ate in the school cafeteria last week.
You randomly select names from the phone book to conduct a survey on technology use.

8. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Margin of Error Formula
When a random sample of size n is taken from a large population, the margin of error is approximated by:
Margin of error =
Which means that if the percent of the sample responding a certain way is p (expressed as a decimal), then the percent of the population that would respond the same way is likely to be between
p – ____ and p + ____.

9. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Example 4
Find a margin of error
Newspaper Survey In a survey of 325 students and teachers, 30% said they read the school’s newspaper every weekday.
What is the margin of error for the survey?
Solution
Margin of error =
The margin of error for the survey is about ______%.

10. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
Example 4
Find a margin of error
Newspaper Survey In a survey of 325 students and teachers, 30% said they read the school’s newspaper every weekday.
Give an interval that is likely to contain the exact percent of all students and teachers who read the school newspaper every weekday.
Solution
To find the interval, add or subtract _____%.
It is likely that the exact percent of all students and teachers who read the school’s newspaper every weekday is between _____% and _____%.

11. Select and Draw Conclusions from Samples
11.4
Select and Draw Conclusions from Samples
c) In Example 4, suppose the sample size is 650 students and teachers. What is the margin of error for the survey?
Is it bigger or smaller? How much?
Is it half?

12. Finally
How many people would you need to randomly survey to get a margin of error of less than 1%?
10,000