Sampling Methods

1
Sampling Methods

• Lesson 3-6

2
Statistics, Data Analysis,and Probability 2.2

• Identify different ways of selecting a sample and
  which method makes a sample more representative
  for a population.
• Convenience sample
• Responses to a survey
• Random sample

3
Key Vocabulary

• Representative
• Bias
• Convenience sample
• Responses to a survey
• Random sample
• Poll

4
Representative

• A random sample which is a good match for the
  population.
• Your results are typical of the population if you
  did get input from EVERYONE or everything.

5
Bias

• A sample which does not mirror the population
• The opposite of representative
• a particular tendency or inclination that
  prevents unprejudiced consideration of a
  question prejudice (from dictionary.com)
• to cause partiality or favoritism
• An unfair influence

6
Convenience Sample

• A form of sampling that is usually biased.
• A sampling method where any convenient method is
  used to choose the sample
• Ask your family members
• Ask your friends
• Ask the people who happen to be around you at the
  time
• The people or items included in the study were
  easy to get to

CONVENIENT easy easily accessible easy to get
to
7
Responses to a Survey

• A form of sampling that is usually biased
• If people are not forced to return them, many
  dont
• Forget about them
• Throw them in the trash
• Usually, the people who answer them have a strong
  opinion one way or the other
• People who are busy dont make the time for them

8
Random Sample

• A sampling method in which every person has an
  equal chance of being chosen to participate
• The way the subjects are chosen makes it more
  likely that their opinions are positive,
  negative, and neutral
• Types of random samples
  
  
  
• Raffleif everyone has one ticket
• Pulling names out of a hat
• Choosing every 5th person from a class roster
• Calling every nth person in a phone book (where n
  is a variable for whichever number you choose)

9
Poll

• a sampling or collection of opinions on a
  subject, taken from either a selected or a random
  group of persons, for the purpose of analysis

10
Lets Practice

• For each of the following situations, answer the
  following questions
• Is the information representative or biased?
• What kind of sampling was used
• convenience sample
• responses to a survey
• random sample

11
A state legislator mails questionnaires about
literacy (being able to read) to all the eligible
voters in her district. Only 12 of the voters
return the questionnaires.
BIASED

• Questionnaires are surveys (biased)
• 12 of a population is a very small amount
• What about the voters who cannot read?

Responses to a survey
12
A school principal questions every student whose
school ID number ends in a 9 to gather student
input about plans for the new library.
REPRESENTATIVE

• Anyone in the school can have a student ID number
  that ends in a 9whether they like to read or not
• All grade levels will probably be represented
  because student ID numbers are issued sequentially

Random sample
13
The staff of the RCMS student newspaper conducts
a poll about students favorite vacations by
interviewing every 10th student listed in the
student directory.
REPRESENTATIVE

• Every 10th student
• Everyone has an equal chance of being number 10

Random Sample
14
The town council polls the members of the civic
orchestra about what type of entertainment the
residents want at the Fourth of July Jamboree.
BIASED

• The civic orchestra is a musical group
• A musical group is a form of entertainment
• People in a musical group probably want music for
  entertainment!

Convenience Sample
15
The school has raised enough money to provide new
uniforms for one of its sports teams. Harry, who
is in charge or conducting a study that will
determine which team gets the new uniforms, polls
the football players after practice for their
opinions.
BIASED

• Football is a sport. They probably want the
  uniforms for themselves
• What about members of the other sports?
• How do the other students who watch the sports
  feel?

Convenience Sample
16
TJs Housecleaning keeps a list of satisfied
customers to use as references. TJ wants to know
how his customers feel about the quality of
service they receive, so he sends a questionnaire
to all the people on his reference list.
BIASED

• The list is of satisfied customers. In order to
  be satisfied, you must not have complaints.
• What about the dissatisfied customers who are not
  on the list? If they are not happy, then they
  definitely have something to say about the
  quality of service!

Response to a Survey
17
Homework

• H3-6

18
A market research team wants to determine the
citys best restaurants, so they poll people as
they leave the gourmet deli.
BIASED

• Chances are very great that if someone is leaving
  a gourmet deli, they chose to go there instead of
  someplace else. They will probably choose that
  place as one of the best restaurants!
• The results would be more representative if they
  asked people someplace that was not a restaurant
  (gas station, Target, etc.)

Convenience Sample
19
The service department sends customer feedback
forms to everyone who has had an oil change
appointment the past month.
BIASED

• What about the people who had an appointment the
  month before? Did someone get fired and they
  dont want to include information about him/her
  in their data?
• Sometimes it takes a bit more than a month to
  decide if you are happy with a job.
• Surveys are biased because it is more than likely
  that most people will not answer them.

Response to a Survey
20
The local news crews ask fans leaving a baseball
game whether or not there should be a limit on
how high a players salary can be.
BIASED

• Baseball fans are likely to have a strong opinion
  about the value of their favorite team members
  salaries.
• If the team won, the fans are probably going to
  vote yes
• If the team lost, disgruntled fans may be
  irrationally negative
• The results are likely to be not neutral.
• What about the fans who watch at home?
• What about people who do not watch sports and are
  unhappy about the salaries of sports players?

Convenience Sample
21
A phone company employee calls every 50th name in
the phone book to find out how satisfied people
are with their long-distance service.
REPRESENTATIVE

• Every 50th name
• Everyone is included here
• All ages
• All economic backgrounds
• All long-distance carriers

Random Sample
22
A principal draws 25 student names out of a hat
and contacts them for their opinions on the new
textbooks.
REPRESENTATIVE

• Drawing names out of a hat
• Every student can be chosen
• Every grade level
• Every GPA

Random Sample
23
A fast food restaurant owner wants to know how
her food compares with the food at the other
fast-food restaurants, so she asks customers for
their opinions after they place their orders.
Biased

• If they liked the other places better, they would
  be eating there and not here.
• What would happen to their food if they said
  something bad after they placed their order?
• Why didnt she ask them after they paid their
  bill?

Convenience Sample