Backtesting technical indicators and viewing historical charts of currencies or stocks, for example, can provide useful information about whether a technical indicator or combination of indicators can be relied upon to help make profitable trading decisions.

However in my years of experience as a forex trader and having spent hours on end poring over historical charts to see how effective a particular indicator or system is, there is one thing I’ve learnt and that’s that historical data can very often be misleading.

Often you will find that the latest technical indicator that you’re testing out has proven to be extremely effective at predicting forthcoming price moves based on historical charts, but when you come to trade this indicator in real time the results are not as profitable as it would seem from your past analysis.

This is because there are certain indicators that repaint data in real time that doesn’t necessarily show up in historical charts. They may change or give a clear signal during a particular candle period, but after the candle or bar is closed, there is no evidence that such a signal ever took place.

This is why real time trading is so much harder than it would seem from analysing price charts from the past.

An example of such an indicator is any of the moving averages. Let’s take the EMA (Exponential Moving Average) as an example.

Often you will see a shorter term EMA cross a longer term EMA in real time, which is very often a strong signal, but if the price suddenly reverses then the shorter term EMA will also reverse and so a crossover may not happen at all.

Therefore when the current candle closes it will appear as if a crossover never actually happened even though in real time it did briefly and you could have made a trading decision based on this crossover. So this is an example of how historical data can be misleading and doesn’t always tell the whole story.

Similarly there are are a number of other repainting indicators which can also change or reverse in real time, but which don’t necessarily indicate this when viewed later on on a historical chart after the candle is closed.

So overall you have to be very careful when viewing past data because often the chart will tell a different story after the candle or bar has closed than what actually happened when you were trading live. If historical patterns and trends played out exactly in real time as they appeared to do in the past, with no misleading or false signals, then we would all be extremely wealthy.

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