Other Sampling Methods
Section 3.1B
Why not SRS?
Sometimes it’s just not feasible or
practical.
Sometimes there are statistical
advantages to using more complex
sampling methods.
Stratified Random Sample
 Divide the population into groups of
individuals that are similar in some way that
is important to the response.
 Choose a separate SRS in each stratum
 Combine these to form the full sample
Example
To survey radio station about the most
requested songs we randomly choose 100
radio stations from each geographic
location.
Cluster Sample
 Divide the population into groups or clusters.
 Randomly choose some of these clusters.
 All individuals in the chosen clusters are the
sample
Example of Cluster
Survey AP students to see if they had enough
time to take the test. We randomly pick some
of the schools that took the test & every
student at the selected schools are surveyed.
Systematic Sampling
This is where you survey every kth person. You
randomize it by randomly choosing where to
start.
 In a large city school system with 20
elementary schools, the school board is
considering the adoption of a new policy
that would require elementary students to
pass a test in order to be promoted to the
next grade. The PTA wants to find out
whether parents agree with this plan.
 Tell what type of sampling was used and
what biases (if any) might result.
 Don’t forget convenience and voluntary.
Put a big ad in the newspaper asking
people to log their opinions on the
PTA web site.
 Randomly select one of the elementary
schools and contact every parent by
phone.
 Send a survey hoe with every student,
and ask parents to fill it out and return
it the next day.
 Randomly select 20 parents from each
elementary school. Send them a
survey, and follow up with a phone call
if they do not return the survey within
a week.
 Run a poll on the local TV news, asking
people to dial one of two phone numbers
to indicate whether they favor or oppose
the plan.
 Hold a PTA meeting at each of the 20
elementary schools and tally the opinions
expressed by those who attend the
meetings.
 Randomly select one class at each
elementary school and contact each of
those parents.
 Go through the district’s enrollment
records, selecting every 40th parent.
PTA volunteers will go to those homes
to interview the people chosen.
 Activity: Sampling Sunflowers
squares using the rows as strata. Then, repeat using the
columns as strata.
Sampling and Surveys
 Use Table D or technology to take an SRS of 10 grid
 Example: Sampling at a School Assembly
methods to select 80 students to complete a survey.
 (a) Simple Random Sample
 (b) Stratified Random Sample
 (c) Cluster Sample
Sampling and Surveys
 Describe how you would use the following sampling
Inference
 It is the process of drawing conclusions about a
population on the basis of sample data.
 Inferences from convenience samples or
voluntary samples would be misleading because
the methods are biased.
Activity
 One at a time, each student will take a SRS of 20 chips and
record the proportion of chips obtained that are red.
 Dotplot of sample proportions:
Trusting Random Samples
 Laws of probability all us to say how likely it
is that sample results are close to the true
population.
 Laws of probability allow trustworthy
inference about the population with a
margin of error.
Margin of Error
 Sets bounds on the size of the likely error.
 Larger random samples give better
information about the population than
smaller samples.
Homework
 Page 227 (17-25) odd
 Page 230 (37-42)