Review
• lurking variable: a variable, usually unobserved, that
influences the association between the variables of primary
interest.
• A lurking variable may be a common cause of both the
explanatory and response variable.
• experiment: assigning subjects to certain experimental
conditions and then observing outcomes on the response
variable.
• treatments: the experimental conditions, which correspond
to assigned values of the explanatory variable.
• observational study (nonexperimental studies):
observing values of the response variable and explanatory
variables for the sampled subjects, without anything being
done to the subjects (such as imposing a treatment)
Data Types in Observational Studies
• anecdotal evidence
• sample survey: selecting a sample of subjects from a
population and collects data from them
• census: a survey for the whole population — expensive, time
consuming or impossible
• sampling frame: the list of subjects in the population from
which the sample is taken
Ideally, the sampling frame lists the entire population.
In practice, it’s usually hard to identify every subject in the
population.
• sampling design: the method for selecting subjects from the
sampling frame
• simple random sample: a simple random sample of n
subjects from a population is one in which each possible
sample of that size has the same chance of being selected
• Use Random Numbers to Select a Simple Random Sample
Methods of Collecting Data in Sample Surveys
• personal interview
• telephone interview
• self-administered questionnaire
Measuring the Accuracy of the Results from Surveys with Random
Sampling
• margin of error
1
approximate margin of error = √ × 100%
n
where n is the sample size.
• bias: certain outcomes will occur more often in the sample
than they do in the population due to the inappropriate way
of the survey
sampling bias
occurs from using nonrandom samples or having
undercoverage
nonresponse bias
occurs when some sampled subjects cannot be reached or
refuse to participate or fail to answer some questions
response bias
occurs when the subject gives an incorrect response
(perhaps lying), or the question wording or the way the
interviewer asks the questions is confusing or misleading
• convenience sample & volunteer sample