If all you know about a stock is its price, you might—and pr...

If all you know about a stock is its price, you might—and probably will—make investing mistakes.
If a stock has had a good run-up, it might be time to sell, not buy, because you want to sell high and re-invest those gains in undervalued stocks.
If a stock has dropped like a rock, it might be a good time to increase your holdings, because the price is liable to rise again once the market settles.

You won't know what to do unless you understand and know a lot more about the company than just its stock price.
What's important in the long-run is a company's fundamentals. Those will always outlast any irrational investor behavior.

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