Compression Ratio
The relationship of cylinder volumes with the piston at TDC and at BDC.

Compression Test
To check of the compression developed in each engine cylinder and is a means of determining engine condition.

Compression ratio is the total volume of the combustion chamber above the piston at the bottom of its travel (BDC), divided by the volume of the combustion chamber above the piston at top dead center (TDC).

Basically all a compression test tells you is the amount of pressure produced in the cylinder by the compression stroke.

See what I'm saying?