POPULATION- the entire group of
individuals that we want
information about
SAMPLE- the part of the population
that we actually examine in order to
gather information about the
population
Population vs. Sample
Gallup finds unemployment at 10.1% in September
The findings were based on more than 18,146 phone
interviews (random digit dialing) with U.S. adults
aged 18 and older
POPULATION: all U.S. adults ages 18 and older
who have telephones
SAMPLE: 18,146 U.S. adults with telephones
SIMPLE RANDOM SAMPLE (SRS)
A SRS (of size n) is selected from a population in a
way that every different possible sample of size n
has the same probability of being selected
Constructing an SRS
Create a list of the sampling frame
Give each object a number
Use a random number table to select sample
An Example: Randomly select 4 students from our
AP Stat class
List all students available for selection
Give each student a number
Since the largest number of students that we
can select only has two digits, we will take
numbers in groups of 2 (start on line 130 of
Table B)
Line 130 of the Random
Number Table
Generating Random Numbers
on the TI Graphing
Calculator
7  Math, rand, enter (change random seed #)
Math >>> 5 (PRB)
randInt (min,max)
Note: If a number is selected more than once, ignore it
Stratified Random Sample
The population is divided into at least two distinct
groups (strata).
A simple random sample of a certain size is drawn
from each strata.
Often, the size is chosen by using the actual percentage
of occurrence in the overall population
Example: Divide class into males and females and
randomly select from each gender in the proportion
that they occur in the population (class).
Think of some other variables that we could stratify
our class by
Cluster Sample
Divide the population into subgroups (clusters)
Select clusters at random
Include all individuals in the selected clusters
in your study
Example: Randomly select homerooms and
then include all students in the homerooms in
the sample
In class, randomly select a row, include all
students in the row to be in the sample
Multi-Stage Cluster Sample
Systematic Sampling
The elements in the population should already be arranged
in some natural sequential order
Choose every kth element in the population to be part of
your sample
List the students in AP Statistics alphabetically
Number the students
Randomly select a student
Choose every kth student depending on the size of your
sample
K = N/n = ________ Round down. Choose every _______
student
All freshman at a university are enrolled in
1 of 30 sections of a freshman seminar
course. To select a sample of freshman at
this university, a researcher selects 4 sections
of the freshman seminar course at random
and all students in the 4 selected sections
are included in the sample.
A) Simple Random Sample
B) Stratified Sampling
C) Cluster Sampling
D) Systematic Random Sampling
To obtain a sample of those attending a
basketball game, a researchers selects
the 24th person through the door.
Then, every 50th person after that is
also included in the sample.
A) Simple Random Sample
B) Stratified Sampling
C) Cluster Sampling
D) Systematic Random Sampling
How can we select a simple random sample
from the previous example?
The NBA could number an alphabetical list of players
from 1 to 348. They could use a random number generator
on a calculator to generate 58 UNIQUE random numbers
from 1 to 348. The players corresponding to those 58
numbers would be asked to participate.