Consequently in order to promote their systems they must either quote short term results or present back-tested performance using historical market data. Performance based on back-testing is notoriously unreliable. The optimum parameters must be found by cut and try methods, i.e. tuning the system to old market data. However successful technical market indicators tend to destroy their own effectiveness with use. Thus it is not surprising that reliable indicators can seldom be found from historical data.
Chance plays the dominant role in short term performance.
One should therefore be wary of reading significance into short term results. Without valid long term records this presents a real dilemma to professional market timers. Fortunately for them, there seems to be no shortage of investors who are persuaded by short term performance. And there will always be another generation of novices who can be lured into market timing by the siren song of professionals. .