News events and earnings reports can change the perceived value of a company. Because the stock market functions as an auction, prices sometimes need to adapt for a trade to occur. When there are more sellers than buyers, the price will go down. Alternately, a stock that has more who want to buy than sell will experience a price increase. Buyers and sellers can be individuals, corporations, asset management companies, or others. Price fluctuations can be dramatic in just one day.
You might hear experienced investors talk about the concept of a "wide moat," especially if you're reading anything about Warren Buffett's investment style. Just as a wide moat around a castle makes it difficult for enemies to invade, a company should have a "wide moat," too: a sustainable competitive advantage that will prevent competitors from stealing that company's market share.
One constant principle of investing is that markets fluctuate. Stock prices will rise and fall for a number of reasons: the economy, investor sentiment, political uncertainty at home or abroad, energy or weather problems, or even corporate scandals. This means market performance isn’t always predictable. That is why diversification, or spreading the investments in your portfolio among different asset classes and across different sectors within each class, is such an important strategy. Diversification is a time-tested way to manage risk.
Even when the stock price has performed as expected, there are questions: Should I take a profit now before the price falls? Should I keep my position since the price is likely to go higher? Thoughts like these will flood your mind, especially if you constantly watch the price of a security, eventually building to a point that you will take action. Since emotions are the primary driver of your action, it will probably be wrong.
In contrast, professional fund managers (information here) do not want tips. They have dozens of good ideas of their own. They won't be sharing those ideas with you and they will not be expecting you to share yours. Instead, they ask about how you allocate money. "Which sectors and markets do you like and why?" The difference between these approaches is like night and day.
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To make comparisons between companies, sectors and markets a little easier, there are a number of mathematical models used. The most common and often the most helpful is the P/E ratio. The Price to Earnings ratio takes the share price and is divided by the earnings per share. It is possible to calculate this using past earnings, projected future earnings and with all sorts of moving averages ;-) Therefore, this is one number that it is vital for any investor to know and understand.
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