Animating particles's count

Explaining better the title and my problem.
I create a plane and a group of a bunch of objects.
I add to the plane a particle system that use the group for rendering.
I activate the option “Use count”.
Now what I’m wondering is if it’s possible to animate the “count” parameter of each single member of the group.
Because if I simply press “i”, I recieve this message: “Failed to resolve path to property, try manually specifying this using a Keying Set instead.”