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¥3dRegAna:
Qualitative Variables (ANCOVA)
HQualitative
variables can also be used
åi.e.,
WeÕre modeling the response amplitude to a stimulus of varying contrast when subjects are either young, middle-aged or old.
åX1 represents the stimulus contrast
(quantitative): continuous covariate
åCreate
indicator variables X2 and X3 to
represent age:
íX2 
=
1 if subject is middle-aged
=
1 if subject is middle-aged= 0 otherwiseíX3 =
1 if subject is old (i.e., at least 1 year older than Bob Cox)
=
1 if subject is old (i.e., at least 1 year older than Bob Cox)= 0 otherwise åFull
Model (no interactions between age and contrast)
íY =
b0 + b1X1 + b2X2 +
b3X3 +
e
E(Y) = b0 + b1X1 for
young subjects
for
young subjectsE(Y) = ( b0 + b2 ) + b1X1 for middle-aged
subjects
for middle-aged
subjectsE(Y) = ( b0 + b3 ) + b1X1 for old subjects
for old subjectsåFull
Model (with interactions between age and contrast)
íY =
b0 + b1X1 + b2X2 +
b3X3 +
b4X2X1 +
b5X3X1 +
e
E(Y) = b0 + b1X1 for
young subjects
for
young subjectsE(Y) = ( b0 + b2 ) + ( b1 + b4 )X1 for
middle-aged subjects
for
middle-aged subjectsE(Y) = ( b0 + b3 ) + ( b1 + b5 )X1 for
old subjects
for
old subjectsHWill
be easier to run analysis in Matlab script for 3dANOVA4, when ready!