PowerPoint Presentation - Using AFNI Interactively

–22–

Multiple Regressors: Collinearity!L!

•Green curve = signal model for #1

•Red curve = signal model for class #2

•Blue curve = signal model for #3

•Purple curve =

#1 + #2 + #3

which is exactly = 1

• We cannot — in principle or in practice — distinguish sum of 3 signal models from constant baseline!L!

No analysis can distinguish the cases

Z(t )=10+ 5×#1 and

Z(t )= 0+15×#1+10×#2+10×#3

and an infinity of other possibilities

Collinear designs

are bad bad bad!