The common ratio effect is another failure of the theory to cope with humans, 
similar to the common consequence effect.

Let:

X={3*10^3}
Px={1}

Y={4*10^3,0}
Py={.8,.2}

X'={3*10^3,0}
Px'={.25,.75}

Y'={4*10^3,0}
Py'={.2,.8}

You should choose X,X' or Y,Y' but this is often violated.