Addicts often have to take heavier doses to get the same thrill as time passes. The same is true in aging bull markets: Companies need to report bigger and bigger earnings to get the same rise out of investors.

What matters to a stock price is not how much profit the company earns, but how much it earns relative to what the market was expecting. In what’s called a positive earnings surprise, a company reports a profit greater than analysts are forecasting. In a negative earnings surprise, the company announces a profit below analysts’ expectations. (Money-losing companies can also surprise, by doing more or less badly than expected.)…

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