Introduction to Sampling
What We Will Learn
 We will learn how to produce data in a manner which
will be trustworthy.
 This will enable us to judge whether conclusions based
on this data are reliable or not.
 Statistical techniques for producing data will allow is
to apply Statistical Inference.
 Statistical Inference – this is the art of using results
obtained from sampling to make inferences about
populations of interest.
Important Definitions
 Population: a population in a statistical study is the
entire group of individuals about which we want
information.
 Sample: a sample is a part of the population from
which we actually collect information used to draw
conclusions about the population.
 Sampling: refers to the process of choosing a sample
from the population.
 A census attempts to contact every individual in the
entire population.
Preliminary Steps for Sampling
 One must state carefully exactly what the population
of interest is.
 One must decide exactly what is to be measured.
 Example: How can a survey measure unemployment?
Bad Sampling Methods
 Convenience Sample: A convenience sample is a
sample of individuals who are selected because they
are members of a population who are most convenient
to reach, such as people passing by in the street.
 Usually, such a sample cannot be trusted to be
representative of the population.
 Example: The Inconvenient Truth about Convenience
Samples.
Bias
 Bias: The design of a statistical study is biased if it
systematically favors certain outcomes.
 When a sampling method is biased, not all the
members of the population in question have the same
chance to be included.
 If certain members of the population are not included
in a study, the results of the study may not generalize
back to the population in question.
 When you encounter the term “bias”, think of
“missing the mark”.
Voluntary Response Sample
 A Voluntary Response Sample consists of people
who choose themselves by responding to a general
appeal.
 Examples: Call-in shows, internet polls, etc.
 Voluntary response samples are biased because people
with strong opinions are most likely to respond.
 Example: Are Online Polls in Line?
Homework
 Worksheet for Sections 7.1 and 7.2.