Most of the market's revenue is completed in a short period of time. Most of the time it is horizontal. The sideways market is actually the best time to stock up.
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That very important to choose long term instead of taking risk with day trading, because you can't be sure on your profit during market fluctuations.
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If we are talking trading here, it could not be profitable for you as a trader if you are for the long term, you may lose or huge amount but it is for a long period of time.
I saw some traders who are very good during bear market, or good in a bull market, vice versa.
So I believe that if you want to be a good trader, you can survive on any types of market, even sideways or bear market.