Margin of Safety concept as applied to common stocks, under normal market conditions.

1. In common stock bought for investment under normal conditions, the margin of safety lies in an expected earning power considerably above the going rate for bonds.
2. Assume, earning power (earning yield) is 9% on the price, and that the bond rate is 4%; then the stock buyer will have an average annual margin of 5% accruing in his favour. Over a ten-year period the typical excess of stock earning power over bond interest may aggregate 50% of the price paid. This figure is sufficient to provide a very real margin of safety – which, under favourable conditions, will prevent or minimize a loss.
3. In many cases, such reinvested earnings fail to add commensurately to the earning power and value of his stock.
4. If such a margin is present in each of a diversified list of 20 or more stocks, the probability of a favourable result under “fairly normal conditions” becomes very large. That is why the policy of investing in representative common stocks does not require high qualities of insight and foresight to work out successfully.
5. If the purchases are made at the average level of the market over a span of years, the prices paid should carry with them assurance of an adequate margin of safety.
6. The danger to investors lies in concentrating their purchases in the upper levels of the market, or in buying nonrepresentative common stocks that carry more than average risk of diminished earning power.

Ref: Intelligent Investor by Benjamin Graham