Topic: Over-optimization (curve fitting)
Over-optimization (curve fitting) is a process of adjustment the indicators' parameters to the concrete data.
Computer technology can be easily used to over-optimize a trading system and produce something that looks good. An optimizer tests thousands of parameters value and can find the exact numbers making the optimizing strategy very "profitable". The problem is that "Profitable" strategy is profitable only applied on this data. However, it is highly possible the system to fall apart in real world.
Strategy parameters that work over a range of values are robust.
If the parameters of an indicator are slightly changed and the performance falls drastically, beware.
For example, if a system works great at MA(20), but does not work at MA(19) or MA(21) you have an over-optimized system.
On the other hand, if your strategy works with Moving Average with a period from 15 until 25, your system is much more robust (and reliable).