What are the limitations of the PEG ratio for equity valuation?

The PEG ratio (Price/Earnings-to-Growth) attempts to normalize the P/E ratio by growth, with the intuition that a stock with a P/E of 30 growing at 30% (PEG = 1.0) is comparably valued to one with a P/E of 15 growing at 15% (PEG = 1.0). While popular, it has significant limitations.

PEG Formula:

> PEG = (P/E) / (Expected EPS growth rate in %)

A PEG of 1.0 is often cited as 'fairly valued,' below 1.0 as cheap, and above 1.0 as expensive.

Limitations:

1. Assumes a Linear Relationship Between P/E and Growth

The DDM shows that the justified P/E is a non-linear function of growth:

> Justified P/E = p / (r - g)

This is a hyperbolic relationship. Doubling the growth rate more than doubles the justified P/E when g approaches r. The PEG ratio assumes it doubles exactly — which is mathematically incorrect.

2. Ignores Risk (Cost of Equity)

Two companies with identical growth but different risk profiles should trade at different P/E ratios. PEG treats them as equivalent.

Same PEG, but Caldwell is riskier — its higher cost of equity means it should trade at a lower P/E. PEG misses this entirely.

3. Undefined or Misleading for Low/Zero/Negative Growth

• Zero growth: PEG = infinity (meaningless)
• Negative growth: PEG becomes negative (nonsensical)
• Very low growth (1-2%): PEG ratios become extremely large and lose comparability

4. Which Growth Rate?

The PEG varies dramatically depending on whether you use 1-year, 3-year, or 5-year growth estimates. There is no standard, and different sources produce different PEGs for the same stock.

5. Ignores the Payout Ratio

The justified P/E depends on the payout ratio (p). A company that pays out 60% of earnings will have a different justified P/E than one paying 20%, even at the same growth rate. PEG ignores this.

6. Does Not Account for the Quality or Sustainability of Growth

Growth from aggressive acquisitions funded by debt is different from organic growth. PEG treats a percentage the same regardless of source.

When PEG Is Useful (With Caveats):

• Comparing high-growth companies in the same industry with similar risk profiles
• As a rough screening tool — not a stand-alone valuation metric
• When combined with other multiples (EV/EBITDA, P/B) for a multi-metric approach

Exam tip: CFA Level II frequently tests PEG limitations. The most testable point is that PEG assumes a linear P/E-to-growth relationship, which contradicts the non-linear DDM formula. If you see a question asking why PEG might give misleading results, start with this.

Deepen your understanding of valuation multiples in our CFA Level II equity course.