# Selection Bias - PowerPoint PPT Presentation

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## Selection Bias

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### The most obvious example of selection bias is when we, knowing what we want to ... Avoid cases of extreme selection bias where there is no variation on the ... – PowerPoint PPT presentation

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Title: Selection Bias

1
Selection Bias
• By Peter Boothby

2
Selection Bias
• How should we select observations for inclusion
in a study?
• The decision as to which observations to select
is important for the outcome of the research and
the degree to which it can produce reliable
results.
• The most obvious example of selection bias is
when we, knowing what we want to see as the
outcome our research, select observations that
support this desired conclusion.

3
• To avoid selection bias, selection should allow
for the possibility of some variation on the
dependent variable.
• For example, research that attempts to explain
the outbreak of war with studies only of wars,
fails to let the dependent variable vary.

4
• When observations are selected on the basis of a
particular value of the dependent variable, we
cannot learn anything about the causes of the
dependent variable without taking into account
other instances when the dependent variable takes
on other values.

5
• We must be aware of the biases introduced by
selection on the dependent variable and seek to
correct for these biases.
• Avoid cases of extreme selection bias where there
is no variation on the dependent variable
because we cannot learn about causal effects.

6
Avoiding Selection Bias
• Selecting cases on the independent variable is
less likely to bias results.
• Avoid bias by selecting on an explanatory
variable that is irrelevant to the study (and has
no effect on the dependent variable).