Price-to-earnings and price-to-sales ratios are the most commonly cited valuation shortcuts in investing, and they are also among the most frequently misapplied. A single number in isolation rarely tells an investor whether a stock is cheap or expensive.

The P/E ratio divides share price by earnings per share, and it works reasonably well for mature, profitable companies with stable margins. It breaks down for cyclical businesses at the peak or trough of their earnings cycle, and it is meaningless for companies with negative earnings, which is why growth investors often reach for price-to-sales instead.

P/S ratios sidestep the profitability problem but introduce a different one: they say nothing about whether a company can ever convert that revenue into profit. A software company trading at 10 times sales with 80 percent gross margins is a very different proposition than a retailer trading at 2 times sales with 25 percent gross margins, even if the multiples look comparable on a spreadsheet.

The more useful exercise is comparing a company’s current multiple to its own historical range, and to a peer group with similar margin structures and growth rates. A stock trading below its five-year median multiple is not automatically cheap; it may simply reflect a legitimate deterioration in the underlying business.

Analysts who focus on valuation gaps, the difference between what a company’s filed numbers support and what the market is currently paying, tend to produce more durable conclusions than those who anchor on a single ratio. Publications like BullScope build entire research notes around exactly this kind of gap analysis, weighing reported fundamentals against the price the market has already assigned.

Used carelessly, multiples become a way to confirm a bias. Used carefully, cross-referenced against margins, growth durability, and the company’s own history, they become one of the more reliable tools an investor has for framing a valuation debate.