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An observational study observes individuals
and measures variable of interest but does
not attempt to influence the responses.
 Often fails due to lurking variables.

An experiment, on the other hand,
deliberately imposes some treatment on
individuals in order to observe their
responses.
 Is the preferred method of gaining knowledge
since it allows us to control lurking variables.
AP Statistics, Section 5.1, Part 1
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The entire group of individuals that we want
information about is called the population.
A sample is a part of the population that we
actually examine in order to gather
information.
The units are the individuals on which the
experiment is performed.
AP Statistics, Section 5.1, Part 1
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Sampling involves studying a part in order to
gain information about the whole.
Sample Frame is the actual set of units from
which a sample has been drawn.
A census attempts to contact every individual
in the entire population.
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Getting a portion of the population is not
difficult.
Getting a good unbiased sample is difficult.
Creating a plan to do this is called “sample
design”.
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Voluntary response sample (example: Call in
opinion polls).
The problem with call in opinion polls is that
the people who answer the polls tend to have
strong opinions, especially strong negative
opinions.
This sample is biased; this sample is not
representative of the population.
AP Statistics, Section 5.1, Part 1
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Convenience sample (example: Mall intercept
interviews)
 Convenience sampling may not get you access to all
the people in the population.
 Interviewers often avoid people who may make
them feel uncomfortable.
 This sample is biased; this sample is not
representative of the population.
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The design of a study is biased if it
systematically favors certain outcomes.
You are conducting an observational study on
how many students in you class like your
math teacher. State if the following is an
example of:
Convenience sampling
Voluntary Response sampling
A good sampling design
Not even sampling
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The easiest way to sample is to use a “simple
random sample”
A simple random sample (SRS) of size n
consists of n individuals from the population
chosen in such a way that every set of n
individuals has an equal chance to be the
sample actually selected.
 Avoids favoritism by the selectors and self-
selection by the respondents.
AP Statistics, Section 5.1, Part 1
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Choose an SRS in two steps:
 Step 1: Label. Assign a numerical label to every
individual in the population.
▪ Use the smallest amount of digits possible.
▪ Each label must contain the same amount of digits.
 Step 2: Random Assignment.
▪ Random number table (Table B)
▪ Random number generator (RandInt in the TI-83)
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A method of selecting members from a larger
population according to a random starting point
and a fixed, periodic interval. Typically, every
“nth” member is selected from the total
population.
Ex: A cop at a checkpoint decides to speed check
every 10th car that passes him.
AP Statistics, Section 5.1, Part 1
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To select a stratified random sample, first
divide the population into groups of similar
individuals, called strata. Then choose a
separate SRS in each stratum and combine
these SRSs to form the full sample.
AP Statistics, Section 5.1, Part 1
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Convenience – it is easier to sample smaller
groups.
Coverage – representation from each group
is assured.
Precision – gives estimates closer to the true
value of the population than an SRS.
AP Statistics, Section 5.1, Part 1
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Good sampling technique uses random
selection to reduce the possibility of bias.
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Undercoverage occurs when some groups in
the population are left out of the process of
choosing the sample.
 Surveys of households leave out homeless people,
prison inmates and college dorms…
 Opinion polls by telephone leaves out people with
out phones, or are not listed…

Nonresponse occurs when an individual
chosen for the sample can’t be contacted or
does not cooperate.
AP Statistics, Section 5.1, Part 1
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Response bias. Respondents may lie if they
feel uncomfortable telling the truth.
 The interviewer’s attitude towards some answers
may influence how the respondents answer.
 Ex. The interviewer wears an anti-abortion shirt
while asking people about their views on abortion.
AP Statistics, Section 5.1, Part 1
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Wording of questions. “It is estimated that
disposable diapers account for less than 2%
of the trash in today’s landfills. In contrast,
beverage containers, third-class mail and
yard wastes are estimated to account for
about 21% of the trash in landfills. Given this,
in your opinion, would it be fair to ban
disposable diapers?”
AP Statistics, Section 5.1, Part 1
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“Do you own or have you ever own a fur
coat?”
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We want to make inferences about the
population as a whole.
We can’t afford to talk to everyone.
Even though two samples, following the
same design most probably will give us
different results, those results are reasonable
estimates of the population as a whole
AP Statistics, Section 5.1, Part 1
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Properly designed samples avoid systematic
bias, but their results are rarely accurate and
vary from sample to sample. (Sampling Error)

Large random sample give more precise
results than smaller sample.
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