How to Read the P/E Ratio — and When It's Lying to Your Face

You open a screener, sort stocks by P/E from lowest to highest, and a thought clicks in your head: "This is at a P/E of 6, that's almost a steal!" Your finger is already hovering over the button. And this is exactly when most people make their first mistake — they confuse a low number with a cheap opportunity. P/E is not a price tag. It's a ratio that can be bent, distorted, and in worse cases, completely turned upside down.

Imagine a sharper version of this scene: you find a stable company with a P/E of 4.2, your heart skips a beat, you buy — and three months later, the company announces a massive accounting write-off, profits vanish, and the price drops by a third. What happened? You fell victim to one of the most common traps in the financial world. Let's break it down by the numbers.

What P/E Actually Measures (and What People Think It Measures)

P/E = stock price ÷ earnings per share (EPS). When a stock costs 100 CZK and the company earns 5 CZK per share annually, the P/E is 20. The popular interpretation is: "How many years it will take for the company to pay for itself at the current profit." P/E 20 → roughly 20 years, if profits never change.

And it's precisely that last "if profits never change" that's the sentence most people skip over — and yet it's where the whole problem is buried.

The indicative long-term average of the American market (S&P 500 index) hovers around a P/E of 15–20. When the index is significantly above this range, historically it has been a signal of higher expectations built into prices; when deeply below it, there were fears. But — and this is crucial — the number itself won't tell you if it's justifiably expensive.

Data Block: Why Low P/E Isn't Automatically a Win

Imagine two companies:

Company A: price 100 CZK, EPS 12.5 CZK → P/E 8. Looks cheap.
Company B: price 100 CZK, EPS 3.3 CZK → P/E 30. Looks expensive.

Now let's add context that the P/E itself doesn't see:

Company A is a cyclical mining company at the peak of the commodity cycle. Last year it had record profits, but raw material prices are falling. In two years, EPS could drop to 4 CZK — and suddenly the "cheap" P/E of 8 is actually a P/E of 25 on normalized earnings.
Company B is growing revenues by tens of percent annually. If profits grow to 8 CZK in three years, today's price corresponds to a future P/E of around 12.

Conclusion? The lower number was more expensive. This is called a value trap — a low P/E because the market rightly suspects that profits will fall.

One-Time Profit as an Optical Illusion

An even more illustrative example. We have two companies with the same market value of 10 billion CZK.

Stability Inc. earns roughly 500 million in net profit annually → P/E 20. Fair valuation of a healthy company.
One-Time Ltd. also earned 500 million last year, but this year it sold an old hall and a patent license, which brought in 2 billion one-time into the accounts. The total profit for this year jumped to 2.5 billion → P/E 4.

At first glance, One-Time is five times cheaper. But next year it will return to standard operating profit and its P/E at the same price will jump back to 20 — or higher if the sale of the factory limits future production. According to historical analyses, approximately 15% of companies in the S&P 500 index in any given year exhibit significant one-time accounting influences (tax effects, write-offs, division sales), which make the current P/E an unreliable indicator.

Four Situations When P/E Lies

1) Cyclical companies at the peak and bottom. For miners, car manufacturers, or steelmakers, the paradox holds: the lowest P/E often occurs just before profits plummet, and the highest (or negative) at the bottom, when recovery is near.

Cycle peak: huge profits -> low P/E (e.g., 5) -> stocks paradoxically MOST DANGEROUS
Cycle bottom: almost zero profits -> high P/E (e.g., 50) -> stocks paradoxically CHEAPEST

That's why for cyclicals, the focus is more on average profit over the entire cycle. Economist Robert Shiller created the CAPE (Shiller P/E) for this reason — instead of last year's profit, it takes the average over the last 10 years adjusted for inflation. Its indicative long-term average for the American market is around 17; when it shot up to around 45 during the dot-com bubble in 2000, it was clear the market was extremely overheated — even though the regular P/E of many companies looked acceptable due to aggressive accounting.

2) One-time items in profit. Sale of a building, revaluation, tax effect — accounting profit jumps, P/E "cheapens" optically, but it's a one-off. That's why adjusted profit without extraordinary influences is also monitored.

3) Share buybacks. When a company buys back its own shares, the number in circulation decreases. Earnings per share (EPS) thus grows optically, even if total net profit stagnates. P/E appears lower and more attractive, without the company selling a single more product. Always ask if "growing EPS" is backed by real business or just buyback math.

4) Difference between trailing and forward P/E. Trailing uses profit from the last 12 months, forward uses analysts' estimates for the next year. For a growing company, forward P/E is lower, for a slowing one, it's the opposite. When you see one enticing number, always find out which it is.

And a small bonus: P/E completely ignores debt. Two companies with the same P/E can be vastly different in risk if one has massive debt to refinance. Here, the supplementary indicator EV/EBITDA helps.

Practical Framework: How to Read P/E in 5 Steps

Before you next say "cheap/expensive," go through this checklist:

Find out if it's trailing or forward. Mixing them up is a classic mistake. Ideally, display both numbers side by side.
Compare with the sector, not the market. A software company typically trades at higher multiples than a bank or utility. Compare apples to apples.
Look at the P/E history of the same company. Is it trading above or below its own five-year average today? That tells more than the abstract "it's a lot."
Pair it with growth — calculate PEG. PEG = P/E ÷ expected annual profit growth in percent. Indicatively: a PEG around 1 suggests the price roughly matches growth. Company B from the example with a P/E of 30 and growth of 30% has a PEG around 1 — suddenly it doesn't look expensive.
Verify the quality of earnings. Is it repeatable? Isn't it inflated by a one-off or buyback? Is free cash flow growing, or just accounting profit? Profit you don't see in the company's account is suspicious.

Small Exercise for Today

Take one company you follow and fill in the table: trailing P/E, forward P/E, five-year average P/E, sector average, expected profit growth. Five numbers. When you have them together, you'll see a story that a single P/E never shows. Most people never do this — and that's why they get lured by the first low number.

In QMA

Compiling these five numbers manually for dozens of companies and reading financial statements for one-offs is a full-time hassle — and that's exactly where the QMA screener saves time. You can filter by P/E, but also display profit growth, sector comparison, and other quality metrics alongside, so you spot the value trap before you fall into it. QMA score doesn't just look at one isolated number, but combines multiple factors together — adjusted earnings, cash flow trends, sector context — which is exactly what P/E alone can't do. It doesn't buy for you, it just serves you context on a silver platter.

Takeaways

P/E is a great starting point and a lousy endpoint. One number without context is like a temperature without knowing if you're measuring a person or a pizza.

Never decide based on P/E alone. Always add growth (PEG), sector comparison, and the company's own history.
Take P/E with a big grain of salt for cyclical and one-time "enhanced" profits — a low number there often means an impending drop, not a discount.
Find out if you're looking at trailing or forward P/E — and don't mix them.
Take one of your companies and fill in the table of five numbers today — you'll know if your previous impression was misleading.

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Disclaimer: This article is for educational purposes only and does not constitute financial advice. Always conduct your own research or consult with a financial advisor before making investment decisions.

📊Is a Low P/E Really a Bargain or a Trap for Unwary Investors?

How to Read the P/E Ratio — and When It's Lying to Your Face

What P/E Actually Measures (and What People Think It Measures)

Data Block: Why Low P/E Isn't Automatically a Win

One-Time Profit as an Optical Illusion

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