Unit Homogeneity

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Unit Homogeneity

• Katie Lipsett
• February 10, 2003

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Why Unit Homogeneity?

• Use estimating causal effects
• Problem with causal inferences
• No time machine
• Cant redo history at the same time and place
  with different values of our explanatory variable
  each time.
• Example Want to test the effect of treatment
  of patients with chicken pox. Cant get chicken
  pox again.

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Whats the next best thing?

• Rerun the experiment in two or more different
  units that are homogenous
• Test more than one person with chicken pox
  assuming homogeneity

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What is unit homogeneity?

• Assumption
• If the units have the same value, then we can
  expect the dependent variable's values to also be
  the same
• How are they homogenous?
• Identical in relevent respects
• Keep validity in mind
• Remember good research methods

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Back to the headaches

• Example
• Two treatments
• Anti-itch cream
• Bath in oatmeal
• Unit homogeneity assumes
• All units (people with chicken pox) are the same
• Effects are measured by treatment (1) treatment
  (2)

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Another example

• Measure the role of incumbency on local school
  board elections
• Choose voting districts that are homogenous
• Cant be perfect
• Assumptions are randomly distributed
• Variables weather patterns, voter registration,
  partisanship, number of candidates, etc.
• Will vary, but try to be the same on average

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Summary of unit homogeneity

• Its an assumption
• Used for causal effect experiments
• Helps to understand the degree of uncertainty or
  likely biases in inferences
• Remember the importance of a good research design

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Works Cited

• King, Gary, Robert O. Keohane, and Sidney Verba.
  Designing Social Inquiry. Princeton Princeton
  Univ. Press, 1994