Many sources of earnings growth cannot legitimately be extrapolated into the future. One must, for example, look at profit margin expansion as a suspicious source of growth. If a company's profit margin expands because it raises its prices, it is probably not reasonable to assume that it can continue to raise prices at the same rate indefinitely. Similarly, if its profit margin expands because it reduces its costs, we might ask how much more it can continue to cut costs without doing serious harm to the company's longer term prospects. Though by keeping its prices at the higher level and by keeping its costs at the lower level, the company may be able to maintain its new level of profitability, this alone will not enable the company to grow to higher levels of profitability.