There are many different test cycles issued by various national and international governments and working groups.
[1] Specified parameters in a test cycle include a range of operating temperature, speed, and load.
But because there is such a wide variety of usage patterns and because it is impractical to test an engine or vehicle under every possible combination of speed, load, and temperature, this may not actually be the case.
Since the latter characteristics are those that attract buyers, a potential problem with cycles that are a simplified version of real-world usage is that vehicle and engine manufacturers may exploit the limited number of test conditions in the cycle by programming their engine management systems to control emissions to regulated levels at the specific test points contained in the cycle, but at all other times to give the best economy or performance.
This undermines the main aim of the tests, namely improved air quality and public health.