When should I use P/E vs. P/B vs. P/S ratios for equity valuation, and what are the pitfalls of each?

Each price multiple has specific strengths and weaknesses. The key is matching the right multiple to the company's characteristics.

Price-to-Earnings (P/E):

• Best for: Profitable, stable companies with consistent earnings
• Strength: Most widely used; earnings drive shareholder value
• Pitfall: Meaningless when earnings are negative. Can be distorted by one-time charges, accounting choices (depreciation methods), or cyclical peaks/troughs
• Trailing P/E uses last 12 months' earnings; Forward P/E uses next 12 months' forecast

Price-to-Book (P/B):

• Best for: Financial companies (banks, insurance), asset-heavy firms, companies with negative earnings
• Strength: Book value is generally positive and more stable than earnings. Useful for firms with large tangible asset bases
• Pitfall: Meaningless for asset-light businesses (software, consulting). Book value may not reflect true economic value if assets are old or intangible-heavy

Price-to-Sales (P/S):

• Best for: Startups, high-growth companies with negative earnings, cyclical firms at earnings troughs
• Strength: Revenue is always positive and harder to manipulate than earnings. Useful when earnings are negative or volatile
• Pitfall: Ignores profitability entirely. A company with $10B revenue and zero margin looks the same as one with 30% margins

Price-to-Cash-Flow (P/CF):

• Best for: Capital-intensive businesses, comparing firms with different depreciation policies
• Strength: Cash flow is harder to manipulate than earnings and strips out non-cash charges
• Pitfall: Different definitions of cash flow (operating CF, free CF, EBITDA) can lead to inconsistent comparisons

Practical Example:

You're analyzing two companies:

• Northwind Software (SaaS, negative earnings, 40% revenue growth) — P/S is most appropriate
• Ironclad National Bank (profitable, large loan book) — P/B is most appropriate

Using P/E for Northwind would produce a meaningless negative number. Using P/S for Ironclad would ignore that banking profitability is the key value driver.

Exam tip: CFA Level I often presents a company profile and asks which multiple is most appropriate. Focus on: (1) Is earnings positive? If no, eliminate P/E. (2) Is the company asset-heavy or asset-light? (3) Is profitability the key differentiator among peers? Also remember that forward P/E is preferred over trailing P/E because valuation should be forward-looking.

Practice more equity valuation with our CFA Level I question bank on AcadiFi.