It really depends a lot on the type of test used. Instant test are not very sensitive and can often times be defeated by drinking LOTS of water before the sample is given. Many of these have a 50 ng detection cutoff. More precise testing such a mass spectrometry and gas chromatography are much more sensitive and can detect as little as a single ng. But most labs that use this technology allow for a certain amount of thc to be present without giving a positive reading. This is usually 20 ng. So a sample could contain 19 ng of thc, an obvious indication of cannabis use, yet still be technically negative. At this point it depends on the policy of the institution who is reading the results. Government agencies are often times more liberal and will not count 19 ng as a positive. This is because of the many lawsuits over the years in which a positive tester has claimed second-hand exposure. Private organizations such as security firms may be much more strict and count any amount as a positive.
But beyond testing procedures, body type is much more important when analyzing the effectiveness of urinalysis. People with high metabolisms and low body fat can consume more drugs and have cleaner drug tests in less time while people with slower metabolisms and more body fat can show positive results for much longer and with smaller amounts used.
The most effective and only universal way of defeating drug tests is to consume large amounts of water before the test. This dillutes the sample (i.e. clear urine). Although this will help a person defeat a drug test it is a method that can be detected using mass spectromtery and gas chromatography. These tests, while primarily designed to detect the presence of drugs, also test the creatine levels, or the concentration of urine in the sample. A low creatine level will indicate a dilluted sample and can invalidate the test. The test would be neither negative or positive and can result in a re-test. But this can be to your advantage as the test results might take several days to complete. On top of this, the institution will have to contact you and schedule a second test which will give you even more time. That is several days more days for your thc levels to go down. Although this avoids a "positive" test, it does indicate potential tampering and strict organizations may penalize you for this this type of test result.