Computers have greatly improved the power and use of mathematical models by performing computations that are very long, very complicated, or repetitive. Therefore computers can show the consequences of applying complex rules or of changing the rules. The graphic capabilities of computers make them useful in the design and testing of devices and structures and in the simulation of complicated processes. The usefulness of a model can be tested by comparing its predictions to actual observations in the real world. A system may stay the same because nothing is happening or because things are happening but exactly counterbalance one another.
Many systems contain feedback mechanisms that serve to keep changes within specified limits.
Different cycles range from many thousands of years down to less than a billionth of a second. As the complexity of any system increases, gaining an understanding of it depends increasingly on summaries, such as averages and ranges, and on descriptions of typical examples of that system. As the number of parts of a system increases, the number of possible interactions between pairs of parts increases much more rapidly.