Why pure resistance value, absorption ratio and polarization index are required for insulation measurement?

For insulation test, the insulation resistance at a certain moment can not entirely reflect the insulation performance of tested object, as the insulation resistance absorbs and polarizes electric charges after high voltage is applied on the insulation material. For insulation tests on sites with main transformers, cables, motors and others, absorption ratio (the ratio between R60s and R15s) and polarization index (the ratio between R10min and R1min) should be measured, so as to determine the insulation performance.