Experiments with a Single Factor

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Experiments with a Single Factor

• Model Adequacy Checking

ModelCheck.ppt
ã D.A. Schaub, Univ. of Florida
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ANOVA Assumptions

• We are assuming a model of
• Therefore residuals (errors) need to be
• Normally distributed
• Independent
• Mean zero
• Constant, but unknown variance s2

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Residuals Examination

• The ANOVA assumptions will not hold exactly, but
  we do need to know about gross violations before
  confirming model validity
• At a minimum we need to calculate residuals
• And then run set validations

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Residuals Calculation Cotton Tensile Strength
Example
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ANOVA Validation Plots

• We need to create plots to validate our
  assumptions
• Normal Probability plot will check for normally
  distributed residuals
• Residuals vs. Fitted Values will check for
  constant variance
• Residuals vs. Run Order will check for
  independence of the residuals
• Minitab will do all of these

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(See figure 3-4, p. 78)
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