Because the data/datafeed (the datafeed is not centralized between the brokers), the charts, the price on the chart, the name of the symbols on the chart, and symbols and the time of the price on the chart - all of them are related to the brokers only.
Forum on trading, automated trading systems and testing trading strategies
Every tick VS Every tick based on real ticks AND latency on StrategyTester (backtesting)
Sergey Golubev, 2017.04.25 18:24
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Of course, if you are planning to trade EA with some broker so you will use broker's data/tick. If you do not like their data so you may change the broker with more good data.
And the data/ticks are not unified for the different brokers ... For example, there are some indicators (iTrend for Brainwashing system for example) which are showing the different results with the different brokers.
There are some EAs (for example: for day trading on D1/W1, martingale) which will have very similar results/performance with the dufferent brokers during the trading. But there are many EAs which are having the performance based on the brokers' data (intra-day trading EAs, scalping, counter trend EAs, and some more).
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If some person will tell me the following: "I have profitable EA."
I will ask him: "Which broker for" (is it profitable for which broker?)
If he will reply: "Any broker" so he is using martingale strategy, or he is trading on W1 timeframe with the trades opened once per half a year for example.
In this way we can understand the recommendation of the MetaQuotes for subscribers concerning the signals to use same broker with the signal provider.
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