The basic method for both the actual measurements and the mathematical calculations behind both standards is similar, however they diverge to a significant degree in the detail, and in the numerical results produced.
To produce a single integer number the measured spectrum is plotted on a graph, and compared against a reference curve (defined in ISO 717-1 for airborne sound insulation, and 717-2 for impact sound insulation).
This is a laboratory-only measurement, which uses knowledge of the relative sizes of the rooms in the test suite, and the reverberation time in the receiving room, and the known level of noise which can pass between the rooms in the suite by other routes (flanking) plus the size of the test sample to produce a very accurate and repeatable measurement of the performance of the sampled material or construction.
It is a measurement which deliberately includes effects due to flanking routes and differences in the relative size of the rooms.
0.5 seconds is often cited as approximately average for a medium-sized, carpeted and furnished living room.
Due to not requiring detailed and accurate knowledge of the dimensions of the test rooms, this index is easier to obtain, and arguably of slightly more relevance.