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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).
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