The ancient Greeks proposed earth, fire, water and air as the main building blocks of all matter, and classified all things as a mixture of these elements. Investing has a similar set of four basic elements that investors use to break down a stock's value. In this article, we will look at the four ratios and what they can tell you about a stock.

Earth: The Price-to-Book Ratio (P/B)
Made for glass-half-empty people, the price-to-book (P/B) ratio represents the value of the company if it is torn up and sold today. This is useful to know because many companies in mature industries falter in terms of growth but can still be a good value based on their assets. The book value usually includes equipment, buildings, land, and anything else that can be sold, including stock holdings and bonds. With purely financial firms, the book value can fluctuate with the market as these stocks tend to have a portfolio of assets that goes up and down in value. Industrial companies tend to have a book value based more in physical assets, which depreciate year after year according to accounting rules. In either case, a low P/B ratio can protect you - but only if it's accurate. This means an investor has to look deeper into the actual assets making up the ratio. (For more on this, see Digging Into Book Value.)


Fire: Price-to-Earnings Ratio (P/E)
The price to earnings (P/E) ratio is possibly the most scrutinized of all the ratios. If sudden increases in a stock's price are the sizzle, then the P/E ratio is the steak. A stock can go up in value without significant earnings increases - this happened most recently in the tech bubble - but the P/E ratio is what decides if it can stay up. Without earnings to back up the price, a stock will eventually fall back down.

The reason for this is simple: a P/E ratio can be thought of as how long a stock will take to pay back your investment if there is no change in the business. A stock trading at $20 per share with earning of $2 per share has a P/E ratio of 10, which is sometimes seen as meaning that you'll make your money back in 10 years if nothing changes. The reason stocks tend to have high P/E ratios is that investors try to predict which stocks will enjoy progressively larger earnings. An investor may buy a stock with a P/E ratio of 30 if he or she thinks it will double its earnings every year (shortening the payoff period significantly). If this fails to happen, then the stock will fall back down to a more reasonable P/E ratio. If the stock does manage to double earnings, then it will likely continue to trade at a high P/E ratio. You should only compare P/E ratios between companies in similar industries and markets. (If these numbers have you in the dark, these easy calculations should help light the way, see How To Find P/E And PEG Ratios.)


Air: The PEG Ratio
Because the P/E ratio isn't enough in and of itself, many investors use the price to earnings growth (PEG) ratio. Instead of merely looking at the price and earnings, the PEG ratio incorporates the historical growth rate of the company's earnings. This ratio also tells you how your stock stacks up against another stock. The PEG ratio is calculated by taking the P/E ratio of a company and dividing it by the year-over-year growth rate of its earnings. The lower the value of your PEG ratio, the better the deal you're getting for the stock's future estimated earnings.
By comparing two stocks using the PEG, you can see how much you're paying for growth in each case. A PEG of 1 means you're breaking even if growth continues as it has in the past. A PEG of 2 means you're paying twice as much for projected growth when compared to a stock with a PEG of 1. This is speculative because there is no guarantee that growth will continue as it has in the past. The P/E ratio is a snap shot of where a company is and the PEG ratio is a graph plotting where it has been. Armed with this information, an investor has to decide whether it is likely to continue in that direction. (Has the P/E ratio lost its luster? The PEG ratio has many advantages over its well-known counterpart, check out Move Over P/E, Make Way For The PEG.)


Water: Dividend Yield
 It's always nice to have a back-up when a stock's growth falters. This is why dividend-paying stocks are attractive to many investors - even when prices drop you get a paycheck. The dividend yield shows how much of a payday you're getting for your money. By dividing the stock's annual dividend by the stock's price, you get a percentage. You can think of that percentage as the interest on your money, with the additional chance at growth through the appreciation of the stock.
Although simple on paper, there are some things to watch for with the dividend yield. Inconsistent dividends or suspended payments in the past mean that the dividend yield can't be counted on. Like the water element, dividends can ebb and flow, so knowing which way the tide is going - like whether dividend payments have increased year over year - is essential to making the decision to buy. Dividends also vary by industry, with utilities and some banks paying a lot whereas tech firms invest almost all their earnings back into the company to fuel growth. (For more read Investment Valuation Ratios: Dividend Yield.)


No Element Stands Alone
P/E, P/B, PEG, and dividend yields are too narrowly focused to stand alone as a single measure of a stock. By combining these methods of valuation, you can get a better view of a stock's worth. Any one of these can be influenced by creative accounting - as can more complex ratios like cash flow. As you add more tools to your valuation methods though, discrepancies get easier to spot. From the Greeks' four basic elements, we now have more than 100, some of which exist so briefly that we wonder if they count, and none of them are named water, earth, air, or fire. In investing, however, these four main ratios may be overshadowed by thousands of customized metrics, but they will always be useful stepping stones for finding out whether a stock's worth buying. (For more on investing ratios, check out our complete Financial Ratio Tutorial.)