Population

Random Selection

Sample
Sampling Vocabulary
• Population of Interest the set of
people or things you wish to know
something about
• Sample those selected randomly to
represent your population of interest
• Random sampling reduces bias when
selecting those from your population of
interest
Sampling Methods
Simple Random
Sample (SRS)
Voluntary
Stratified Random Response Sample
Sampling
Convenience
Systematic Sampling Sample
Judgment Sample
“Quickie Polls”
Simple Random Sampling
From the entire population
every unit has the same chance of
belonging to the sample
and every possible grouping of
specified size has same chance of
being selected.
Using a Random Number Table to get an SRS
• 19223 95034 05756 28713 96409
12531 42544 82853 73676 47150
• Assign a number label to each unit in
the population
• Read numbers from table from left to
right, starting anywhere. The subjects
selected for the sample are those read
from the table.
• Repeats or those not a part of the list
are ignored.
Stratified Sample
1st divide population into groups
(strata), then take a Simple
Random Sample from each
strata
Systematic Sample
From a list of the population,
randomly choose a starting
subject, and then take every nth
subject (for example, start at the
4th person and sample every
10th person after, so #4,14, 24,
34, etc.)
Biased Samples
• Voluntary Response Sample--radio station
call-in or magazine/internet write in
• Convenience Sample--surveying folks in a mall
who appear willing to talk to you
• Judgment Sample – surveying those you pick
as an “expert” selector
• “Quickie Polls”--hastily designed, poorly pretested, one night survey sample for evening
news show