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  Lecture14_OneWayANOVA (3)
  
  # Lecture14_OneWayANOVA (3)
  
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  05/07/2011
  
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  What type of therapy is most
  
  effective in curing phobias?
  
  ±General counseling
  
  ±Systematic desensitization
  
  ±Counter conditioning
  
  Comparing 3 (or more) groups
  
  The IV is nominal (3 categories)
  
  ±Regression requires an ordered (ordinal)
  
  IV so we can¶t use that
  
  Comparing 3 (or more) groups
  
  The IV is nominal (3 categories)
  
  ±Regression requires an ordered (ordinal)
  
  IV so we can¶t use that
  
  ±If we used a t-test, we would have to do
  
  three comparisons:
  
  General counseling vs. Systematic
  
  desensitization
  
  General counseling vs. Counter conditioning
  
  Systematic desensitization vs. counter
  
  conditioning
  
  ±What¶s the problem with this?
  
  Comparing 3 (or more) groups:
  
  Problem with t-test approach
  
  Whenever you do more than one
  
  comparison in an experiment, you increase
  the chance of a Type I error
  
  ±If we used the t-test approach, we would
  
  compare
  
  
  
  1&
  
  2 with = .05
  
  
  
  2&
  
  3 with = .05
  
  
  
  1&
  
  3 with = .05
  
  ±So you would end up with an experiment-wise
  
  error rate of .05*3 = .15
  
  ±Increasing the number of groups makes this
  
  much worse
  
  One-way ANOVA
  
  A ³one-way ANOVA´ is an
  
  experimental design with one
  
  independent variable that represents
  
  more than 2 levels of groups
  
  One-way ANOVA
  
  A ³one-wayANOVA´ is an experimental
  
  design with one independent variable
  
  that represents more than 2 levels or
  
  groups
  
  One-way ANOVA
  
  A ³one-wayANOVA´ is an experimental
  
  design with one independent variable
  
  that represents more than 2 levels or
  
  groups
  
  ±The IV in an ANOVA is called a factor
  
  E.g., Type of therapy, grade level in school (2nd
  
  ,
  
  3rd
  
  , or 4th
  
  ), dosage of a drug (0mg, 100mg, 500
  
  mg)
  
  One-way ANOVA
  
  A ³one-wayANOVA´ is an experimental
  
  design with one independent variable
  
  that represents more than 2 levels or
  
  groups
  
  ±The IV in an ANOVA is called a factor
  
  E.g., Type of therapy, grade level in school (2nd
  
  ,
  
  3rd
  
  , or 4th
  
  ), dosage of a drug (0mg, 100mg, 500
  
  mg)
  
  ³ANOVA´ stands for ³analysis of variance´
  
  Analysis of Variance
  
  When you break up people into groups
  
  (e.g., types of therapy), there are two
  
  sources of variance that are important
  
  Analysis of Variance
  
  When you break up people into groups
  
  (e.g., types of therapy), there are two
  
  sources of variance that are important
  
  ±Variance within each group
  
  Even in the same type of therapy, people will
  
  differ on how much improvement they make (for
  
  This is the noise
  
  Analysis of Variance
  
  When you break up people into groups (e.g.,
  
  types of therapy), there are two sources of
  variance that are important
  
  ±
  
  Variance within each group
  
  
  
  Even in the same type of therapy, people will differ on
  how much improvement they make (for reasons we
  
  This is the noise
  
  ±Variance between each group
  
  We are really interested in the differences between the
  
  groups±does one group improve more than the other
  two?
  
  The variance between the groups is our ³signal´
  
  Analysis of Variance
  
  In some ways, ANOVA should be called
  
  ³analysis of sums of squares´
  
  ±SS
  
  total= Ss
  
  between-groups+ SS
  
  within-groups
  
  The point is to parse out how much of the
  
  variance is happening between the
  
  groups (signal that the group is making a
  
  difference), and how much is happening
  
  within the group (random error, or noise)
  
  scribd
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