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How should I decide whether to buy more shares of a stock when its price drops?
A key principle of economics is: Sunk costs are irrelevant. You bought the stock at 147 and it has now fallen to 144. That's too bad. This has nothing to do with whether it is wise or foolish to buy shares at 144. The only relevant thing to consider is: Do I expect the stock to go up or down from 144? You have lost $3 per share on the original buy. Buying more shares will not "reduce your loss" in any way. Suppose you bought 100 shares at 147. The price then drops to 144. You have lost $3 per share, or $300 total. You buy another 50 more shares at 144. The price stays at 144. So your average purchase price is now (147 x 100 + 144 x 50) / 150 = 146. So I guess you could say that your "average loss per share" is now only $2. But it's $2 x 150 shares instead of $3 x 100 shares. You still lost $300. You didn't reduce your loss by a penny. Maybe it made you feel better that you reduced your average loss per share, but this is just an arithmetic game. If you believe that the stock will continue to drop, than buying more shares just means you will lose even more money. Your average loss per share may go down, but you're just multiplying that average by more and more shares. Of course if you believe that the stock is now at an unjustifiably low price and it will likely go back up, then sure, buy. If you buy at 144 and it goes back up to 147, then you'll be making $3 per share on the new shares you purchased. But I repeat, whether or not you buy more shares should have nothing to do with your previous buy. Buy more shares if you think the price will go up from the present price; don't buy more shares if you don't think it will go up. The decision should be exactly the same as if you had never previously bought shares. (I'm assuming here that you are a typical small investor, that you not buying enough shares to have any significant effect on the market, nor that you are in a position to buy enough shares to take control of the company.)