1.5 Bias in Sampling
If the results of the sample are not representative of the population, then the sample has
bias.
Sampling Bias:
The technique used to obtain the sample’s individuals tends to favor one part of the
population over another.

Any convenience sample has sampling bias.

Sampling bias also results due to undercoverage, which occurs when the
proportion of one segment of the population is lower in a sample than it is in the
population.
Nonresponse Bias:
When the individuals in the sample who do not respond have different opinions than the
ones who do.

Response rates in random digit dialing telephone surveys are typically around
70%.

E-mail survey responses are around 40%.

Mail surveys can have response rates as high as 60%.

Two methods of improving nonresponse are callbacks and rewards for survey
completion.
Response Bias:
The answers on a survey do not reflect the true feelings of the respondent.
There are several reasons that response bias may occur:

Interviewer Error – An experienced interviewer must make the interviewee feel
comfortable enough to answer truthfully.

Misrepresented Answers – May occur when individuals are asked to self-report.

Wording of Questions – Care should be taken to created balanced questions. Also,
avoid vague questions.
o

How could we reword the following question to make it balanced?
Do you oppose the implementation of Obamacare?
Ordering of Questions or Words – A strategy to deal with this is to rearrange the
order of the questions within a questionnaire so that responses are not affected by
prior questions.
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
Type of Question – Open response questions versus closed response questions
such as multiple choice.
What are some advantages and disadvantages to each?

Data-entry Error – Not technically a result of response bias, but can lead to results
that do not represent the population.
Errors in Sampling:

Nonsampling errors result from the process of obtaining the data. These result
from undercoverage, nonresponse bias, response bias, or data-entry error.

Sampling errors result from using a sample to estimate information about a
population. These occur when the sample gives incomplete information about a
population.
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