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At one level, yes. But there are invisible forces which have a far greater influence on profits than the system ever could.
[edit out qldfrog - different systems]
An interesting reply in that it is an answer without an answer.
I will clarify my position:
A backtested system is simply a time series of stock prices [or any prices] to which trading rules are applied, looking for a positive [profitable] outcome.
Clearly the selection of the time series is crucially important. Data should be used from the 1929-1933 debacle, the 1969-1975 period, the 1987 crash, the 1997 currency crisis and the 2008 experience. Of course how that data is aggregated with longer timeframes will have an impact.
At the end of your testing you have a model of how your system [hypothesis] will perform through past historical events, based purely on a price time series. It is a valid exercise because if going forward in real time there is a significant deviation from the model, you can say the model is falsified.
The prices incorporate every event that actually happened that had an effect on the data [price/time series].
Essentially what your answer implies, is (a) for any given event, there are multiple alternative pathways that could have pertained, but didn't, but could the 'next time', or (b) there will be a new event that has not once happened in the existing time series.
With regards to (b), I'll cite the Bible to you, "there is nothing new under the sun".
With regards to (a), a much more interesting proposition and one with which I agree. You could have the same type of trigger [event] with a totally different reaction through prices [human reaction]. In other words, an alternative history plays out.
But: because your outcomes [price time series] are fixed to only 4 possible outcomes, up, down, sideways or complete failure [bourse closes with 100% loss], my question is: do alternative paths make any practical difference?
You may of course in your answer be considering something completely different, which is why enigmatic answers can create misunderstandings.
jog on
duc