SECTION 1.5 - BIAS IN SAMPLING
Definition
If the results of the sample are not representative of the population, then the
sample has bias.
Three sources of bias in sampling:
1.
Sampling bias - this type of bias occurs when the technique used to obtain the
individuals to be in the sample tends to favor one part of the population over
another.
• convenience sampling
• undercoverage - this occurs when the proportion of one segment of the
population is lower in a than it is in the population.
2.
Nonresponse bias - this exists when individuals selected to be in the sample that
do not respond to the survey have different opinions from those who do.
3.
Response bias - this type of bias exists when the answers on a survey do not
reflect the true feeling of the respondent. This can occur be cause of
• misrepresented answers
• wording of the questions
• order of the questions or words
• type of question - closed vs. open
• data entry error
Example 1
To determine the public's opinion of the police department, the police chief
obtains a cluster sample of 15 census tracts within his jurisdiction and samples all
households in the randomly selected tracts. Uniformed police offices go door to door to
conduct the survey. Determine the type of bias.
Example 2
Determine the type of bias is present in the polls mentioned in the Poll
Bearers segment from The Daily Show?
Example 3
The following questions are from a survey sent out by a representative to has
constituents. Determine the type of bias.
7. Which recommendations do you support to balance the state budget.
(Circle all that apply)
a. Privatize costly government programs
b. Reduce the number of people directly receiving government services
c. Reduce state tax rates to increase private investment and business
output
d. Eliminate ineffective stat programs to cut spending
e. Increase gas taxes
f. Reduce program expenditures for noncitizens
g. Modest decrease in state spending balanced by modest fee increases
h. Legalize casino gambling
i. Institute a state income tax
j. Limit future government growth to a rate of population plus inflation
8. Which recommendations do you oppose to balance the state budget.
(Circle all that apply)
a. Privatize costly government programs to reduce the number of people
directly receiving government services
b. Reduce state tax rates to increase private investment and business
output
c. Eliminate ineffective stat programs to cut spending
d. Increase gas taxes
e. Modest decrease in state spending balanced by modest fee increases
f. Legalize casino gambling
g. Institute a state income tax
h. Increase tobacco taxes
i. Increase user fees (i.e. drivers' license fee, auto registration, etc.)
j. Limit future government growth to a rate of population plus inflation
Definition
Nonsampling errors are errors that result from undercoverage, nonresponse bias,
response bias, or data-entry error. Such errors could also be present in a complete census
of the population. Sampling error is the error that results from using a sample to estimate
information about a population. This type of error occurs because a sample gives
incomplete information about a population.