Great discipline is required in following a particular market timing formula. When losses occur, as they inevitably will, there is a strong tendency to abandon the formula. This often makes things worse than they otherwise would have been. There are few, if any, professional market timers with successful long term records. 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.