Examples of types of bias in sampling include:
Bias resulting from incompleteness of the sampling frame: accessibility bias, seasonability
bias, self-reporting bias, volunteer bias, non-response bias etc.
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Non-response bias refers to failure to obtain information on some of the subjects included in
the sample to be studied. It results in significant bias when the following two situations are
both fulfilled.
1. When non-respondents constitute a significant proportion of the sample.
2. When non-respondents differ significantly from respondents.
The issue of non-response should be considered during the planning stage of the
study:
a) Non-response should be kept to a minimum. E.g. below 15%
Methods that may help in maintaining non-response at a low level could be:
• Training data collectors to initiate contact with study subjects in a respectful way and
convince them about the importance of the given study (this minimizes the refusal type
of non-response)
• Offering incentives to encourage participation (this should be done by taking account
of the potential problems that may arise in conducting future research)
• By making repeated attempts (at least 3 times) to contact study subjects who were
absent at the time of the initial visit.
b) The number of non-responses should be documented according to type, so as to
facilitate an assessment of the extent of bias introduced by non-response.
c) As much information as possible should be collected on non-respondents, so as to see
in what ways they may differ from respondents.
• Selection bias cannot be corrected by increasing the size of the sample, why? How
do you remove this type of bias?
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