¥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): 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)
=
1 if subject is old (i.e. at least 1 year older than Bob)= 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,+
b4X2,X1 +
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
available and easier to run analysis in Matlab script