On what basis do you buy, hold, sell ETFs?

I am in the process of building out an investment tracking dashboard using Claude. It is coming along quite nicely. I am trying to see if I can include automated buy, hold, sell recommendations based on objective criteria. I am curious how people here think about such decisions. Is it about having a thesis and checking to see if that thesis is still valid? Is it about excess returns vs. some benchmark? Is it about hitting some predetermined threshold +/-? I realize it can be any one of those or

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