by 6 percent a year over more than two decades. Keynes used many similar terms and concepts as Graham and Dodd (e.g. an emphasis on the intrinsic value of equities). But a review of his archives at King's College found no evidence of contact between Keynes and his American counterparts so he is believed to have developed his investing theories independently, and did not teach his concepts in classes or seminars as did Graham and Dodd. While Keynes was long recognized as a superior investor, the full details of his investing theories were not widely known until decades after his 1946 death. Furthermore, while there was "considerable overlap" of Keynes's ideas with those of Graham and Dodd, their respective ideas were not entirely congruent. Benjamin Graham Value investing was established by Benjamin Graham and David Dodd, both professors at Columbia Business School and teachers of many famous investors. In Graham's book The Intelligent Investor, he advocated the important concept of margin of safety — first introduced in Security Analysis, a 1934 book he co-authored with David Dodd — which calls for an approach to investing that is focused on purchasing equities at prices less than their intrinsic values. In terms of picking or screening stocks, he recommended purchasing firms which have steady profits, are trading at low prices to book value, have low price-to-earnings (P/E) ratios, and which have relatively low debt. Further evolution However, the concept of value (as well as "book value") has evolved significantly since the 1970s. Book value is most useful in industries where most assets are tangible. Intangible assets such as patents, brands, or goodwill are difficult to quantify, and may not survive the break-up of a company. When an industry is going through fast technological advancements, the value of its assets is not easily estimated. Sometimes, the production power of an asset can be significantly reduced due to competitive disruptive innovation and therefore its value can suffer permanent impairment. One good example of decreasing asset value is a personal computer. An example of where book value does not mean much is the service and retail sectors. One modern model of calculating value is the discounted cash flow model (DCF), where the value of an asset is the sum of its future cash flows, discounted back to the present. Quantitative value investing Quantitative value investing, also known as Systematic value investing, is a form of