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for example in stock and forex, people monitor DAW Jones, S&P 500, NASDAQ and ....
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Probably the best suggestion for an index in the crypto area already came from mk4. The bitcoin price regulates the market, and if it falls, almost all other coins fall as well.
On the other hand, if the bitcoin price rises, this does not necessarily mean that other coins will also rise. To mitigate this effectt, you can take a look at the indices for the top 5 or the top 10 coins (by marketcap). For example, the Austrian exchange Bitpanda has such indices listed, mainly for the investment in these coins. On the exchange itself, however, you can also see the price of the respective indices:
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