Ben Graham actually did not publish a formula like this. But he wrote in The Intelligent Investor (1948 version) regarding to the criteria for purchases:
Current price should not be more than 15 times average earnings of the past three years
Current price should not be more than 1.5 times the book value last reported. However, a multiplier of earnings below 15 could justify a correspondingly higher multiplier of assets. As a rule of thumb we suggest that the product of the multiplier times the ratio of price to book value should not exceed 22.5. (This figure corresponds to 15 times earnings and 1.5 times book value. It would admit an issue selling at only 9 times earnings and 2.5 times asset value, etc.)
Unlike valuation methods such as DCF or Discounted Earnings, the Graham number does not take growth into the valuation. Unlike the valuation methods based on book value alone, it takes into account the earnings power. Therefore, the Graham Number is a combination of asset valuation and earnings power valuation.
In general, the Graham number is a very conservative way of valuing a stock. It cannot be applied to companies with negative book values.
Please keep these in mind:
1. Graham Number does not take growth into account. Therefore it underestimates the values of the companies that have good earnings growth. We feel that if the earnings per share grows more than 10% a year, Graham Number underestimates the value.
2. Graham Number punishes the companies that have temporarily low earnings. Therefore, an average of earnings makes more sense in the calculation of Graham Number.
3. Graham Numbers underestimates companies that are light with book.
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