Another commonly used test of sheet metal is hardness testing. This is a relatively simple, fast, and affordable source of information about material mechanical characteristics. Hardness is a material's ability to resist permanent plastic deformation, usually tested by indentation. Hardness testing is not a test of a sheet metal's properties. It is an empirical test of observed results from tests defined under one of a number of standards. As a result, the test standard must be conspicuously identified on the test report. Different test tools, displacements, and forces are used to test small samples of bulk product, surface treatment processes, surface treatments, finely microstructured materials, multi-phase materials, or non-homogeneous materials. Hardness testing is appropriate to measure resistance to indentation, but offers little information about sheet forming differences.