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.
However, it might be best to not become too much of a market "expert". Some of the most famous and successful investors of all time, such as Peter Lynch, the famed manager of the huge Fidelity Magellan fund. He suggested that looking for clues in normal life is a great way to find opportunities. Lynch used to closely follow the shopping habits of his wife to see what brands people were buying. He believed that most people working professionally on the NYSE lived in a bubble.
Diversification allows you to recover from the loss of your total investment (20% of your portfolio) by gains of 10% in the two best companies (25% x 40%) and 4% in the remaining two companies (10% x 40%). Even though your overall portfolio value dropped by 6% (20% loss minus 14% gain), it is considerably better than having been invested solely in company E.

9. Keep a trade journal – Keeping a record of previous trades is an invaluable tip. Software now enables you to quickly and easily store all your trade history, from entry and exit to price and volume. You can use the information to identify problems and amend your strategy, enabling you to make intelligent decisions in future. You never meet a trader who regrets keeping a trading journal.
Why I’m buying: Spell out what you find attractive about the company and the opportunity you see for the future. What are your expectations? What metrics matter most and what milestones will you use to judge the company’s progress? Catalog the potential pitfalls and mark which ones would be game-changers and which would be signs of a temporary setback.