population
Definition:
The whole group that is being
studied.
Example:
All seventh-graders in Cobb
County
Samples and Populations
A sample in a research study is a relatively small number of
individuals about whom information is obtained. The larger
group to whom the information is then generalized is the
population.
Sample
Population
Sampling
Example:
All seventh-graders at
Lovinggood Middle School
Census
•
A census is a survey which measures an entire
population.
Example: The United States conducts a census of the
American population every ten years.
• A census is very tedious and takes a long time to
complete. Therefore, we use samples of a population to
make inferences about that population.
• Inference: Inferences are made when you use clues from
samples to make generalizations about a population.
Why use samples?
Although the best data comes from studying
an entire population, samples are used
because they are smaller and less clumsy to
work with. It can be too time consuming
and expensive to study an entire population.
Random vs. nonrandom
sampling
Random sampling A sample in which each member of the
population has a random chance of being chosen. A random
sample is a better representation of a population than a
non-random sample.
Example: Have a computer select every tenth name from
and alphabetical list of each seventh-grader in Cobb County.
In a nonrandom sample, members are selected on the basis
of a particular set of characteristics, rather than a random
chance of being included.
Example: The first 30 students with red hair are surveyed to
determine if students like math.
Simple random sample
In a simple random sample, each and every
member of a population has an equal and
independent chance of being selected.
Example: Put every student’s name on a piece of
paper, then draw one of the names from the
group.
Other Random Sample Selection Methods
Stratified random sampling from sub-groups in the population
Example: to have a random sample of 100 people evenly divided by gender,
you would divide population into male and female groups and randomly select
50 from each group.
Proportional sampling to insure maintenance of sub-group proportions
Example: divide the population of DoGood Middle School into male and
female groups. If there are 3 girls to every boy, in order to have a random
sample of 100 people balanced on gender we need to randomly select 75 girls
and 25 boys.
Systematic sampling - drawing every 10th person.
Example: to get a random sample of voters you select every 10th person from
the Voter Registration Roles at the courthouse.
Convenience sample
When it isn’t possible to draw a random or systematic
nonrandom sample, a researcher might choose to study
the individuals who are available. This is known as a
convenience sample.
Example: If you wanted to know which movie most
people want to see, you mi9ht just ask your friends
because they are available. However, this is not
representative of the entire population of Georgia.
Biased sample
A sample that does not truly represent a
population.
Example: All of the seventh-graders in Cobb
County who are enrolled in honors English
classes.
Representative samples
• It is important to get a representative sample of the
population in order to provide the most accurate
portrayal of the population being studied.
• Representing sub-groups proportionally is one way to
insure a representative sample
• Generally, the larger the sample, the more
representative of a population.
Using a ____________ sample
increases the chances of collecting
a representative sample.
A.) Biased
B.) Convenience
C.) Random
D.) Narrow
Melissa is collecting data to determine the most
popular type of car driven by people at her company.
Which of the following is an example of a random
sample?
A.) Melissa surveys all of the managers at her company.
B.) Melissa uses a computer to randomly choose 20 people
from a database that includes every person at her company.
She then surveys those 20 people.
C.) Melissa surveys the first 20 people who walk
in the door in the morning.
D.) None of these are random.
A sample where the easiest
method is used to collect data is
known as a ___________ sample.
A.) random
B.) convenience
C.) biased
D.) representative
Your teacher is conducting a survey to
determine the average age of students in your
class. Which of the following would most likely
not result in a representative sample?
A.) Your teacher writes everyone's name down on a piece of
paper and draws 10 names from a hat to survey
B.) Your teacher chooses only students wearing a red or blue
shirt to survey.
C.) Neither of these would result in a representative sample
D.) Both of these would result in a representative sample