Many techs do not use the compression test as a diagnostic tool as there are numerous features on most scopes to replace it. Burn time is perhaps the most used diagnostic tool as it will also show problems with the spark and injectors as well as a weak cylinder. In many instances the check engine light will be on and a code will tell us the problem cylinder. The cylinder balance test is also a replacement for a compression test and is usually followed by a leak down test on the offending cylinder. A Leak down test pumps air into the cylinder and measures the per cent of leakage and by where the pressure is escaping you can tell if it is an exhaust, intake valve or piston problems. When spending some time in some various shops I was also surprised at how many techs use the good old vacuum gauge or the electronic equivalent of one, the electronic one can be graphed and gives the tech a lot of diagnostic info. Also many still use the wet and dry test to check rings and engine condition. The wet dry test is when you check the compression on a dry cylinder and then give the cylinder a couple squirts of oil and check the compression again if it is higher you have a ring sealing problem. The problem here is that many engine compartments prevent access to give the cylinder a squirt of oil. I have seen some vey inventive hose arrangements to try get oil into the cylinder and they usually end up making a mess. Most DIY do not have the diagnostic equipment so they have to rely on the good old compression test. Compression should be within 20% of other cylinders; unless they are all even and one is low then I would like to see less than a 10% difference. This will set a check engine light. Compression gauges are not known for their accuracy that is why the difference is more important than the actual reading. If the gauge fails to hold pressure replace the schrader valve at the end of the hose. It looks like a tire valve but it is not but one can be used in a pinch.