Base Rates: Using Outside-View Statistics to Temper Your Forecasts

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You have just read a persuasive analysis of a young company, and a number appears in your head: it could double within two years. Before you act on it, ask a different question. Of all companies that looked like this one at a similar stage, what usually happened? That is the base rate, and using it to temper your forecasts means starting from the typical outcome for a whole class of cases and adjusting from there, instead of building a forecast from the story in front of you alone.

Key Takeaways

• A base rate is how often something happens across a broad reference class, and it is the right starting point for any forecast.
• The inside view, which focuses on the specifics of your case, tends to produce confident forecasts that are too optimistic.
• The outside view starts with the reference class and adjusts for what is truly different about your case.
• Choosing the reference class is the hard part, and different reasonable classes can give different answers.
• Base rates describe tendencies, not outcomes, so they temper a forecast without replacing judgment.

The Inside View and the Outside View

Psychologists Daniel Kahneman and Amos Tversky distinguished two ways of forecasting. The inside view looks at the specifics of the case in front of you: the management team, the product, the plan, the momentum. It feels natural and rich, and it produces a coherent story. The outside view ignores the story at first and asks what happened to comparable cases in general.

The inside view has a known weakness. When people build a forecast from the details of a particular project, they tend to underestimate delays, costs and obstacles, because the details they can imagine are the ones that go well. The planning fallacy, as it is called, shows up in home renovations, software projects and corporate expansions alike.

Investing is full of inside-view temptations. A compelling business plan, an energetic chief executive and a growing market all feel like evidence. They are, but they are evidence about this company, and they say little about how often companies with such features actually fulfil their promise.

What a Base Rate Looks Like in Practice

A base rate is a frequency across a reference class. Take a few investor-relevant examples. How often do very fast-growing companies sustain their growth rates for many years? How often do companies that announce an acquisition end up creating the value their executives projected? How often do analysts' earnings estimates turn out close to actual results?

You will not always find a clean number, and you should be wary of any figure you cannot trace to a source. But even a rough, honest sense of the typical pattern is useful. Growth tends to fade as companies grow larger. Forecasts tend to be more optimistic than results. Most newly public companies do not become the next household name. These regularities are well documented in finance research, even when the exact percentages vary by period.

Where you cite data, prefer primary sources: company filings on the SEC's EDGAR database, and investor-education material from the SEC and FINRA. Be skeptical of statistics that arrive without a source or a definition of the group they describe.

Choosing the Reference Class

This is where the method gets difficult. A reference class is the group of cases you decide your situation resembles, and reasonable people can choose differently. Is a given company one of all small companies, all small companies in its industry, or all small companies in its industry at the same stage of growth? Each narrower class is more relevant but contains fewer cases, so the estimate becomes noisier.

A practical approach is to try two or three plausible classes and see how much the answers differ. If they agree, you can lean on the result. If they diverge widely, that spread is itself information: it tells you your uncertainty is large, and your forecast should be humble.

Be honest about motivated reasoning here. It is easy to pick the reference class that makes your preferred conclusion look typical. Write down the class before you look at what it implies, so the choice is made before your hopes can steer it.

Adjusting From the Base Rate

The base rate is a starting point, not a verdict. After anchoring on it, adjust for features that truly distinguish your case, and be disciplined about what counts. A genuine, documented difference, such as a business model with unusually low costs, can justify moving away from the typical outcome. A general feeling of confidence cannot.

A good habit is to adjust in modest steps and to say out loud why each step is warranted. If you find yourself moving far from the base rate on thin evidence, treat that as a warning. The strongest-sounding stories are the ones that most tempt investors to leave the average behind.

Finally, remember that a base rate describes a tendency across many cases. It cannot tell you what will happen in any single one. A company can beat the odds, and a typical outcome can still fail to materialize. The aim is to hold forecasts with the right amount of confidence, not to predict.

An Illustrative Example

This is an illustration, not a recommendation or a result. Imagine a physician who has read a glowing account of a small company with a promising product and finds herself forming a confident forecast of rapid growth.

She pauses and writes down the reference class: small companies in this field at a similar stage. She reads about how such companies have typically fared, using filings and general research, and concludes that most struggle to sustain early growth and many fall short of their initial plans. She tries a second class, small companies in general, and finds a similar pattern.

She does not abandon the idea. She lowers her confidence, writes what specific evidence would justify moving above the typical outcome, and decides on a small position size, or no position, until that evidence appears. Her forecast is now an honest range rather than a single bright number.

Frequently Asked Questions

What is a base rate in investing?

It is how often an outcome occurs across a broad group of similar cases, such as how often fast-growing companies keep growing at that pace. It gives you a starting point before you consider the specifics of one company.

Why do investors ignore base rates?

Vivid stories about a single company feel more convincing than dry statistics about many companies. Psychologists call the tendency to focus on specifics base-rate neglect, and it is a well-studied bias.

What is the difference between the inside view and the outside view?

The inside view forecasts from the details of the case in front of you, while the outside view starts from how comparable cases generally turned out. Using both, with the outside view first, tends to temper over-optimistic forecasts.

Can base rates predict what a stock will do?

No. They describe tendencies across many cases and cannot tell you the outcome of any single one. Their value is in keeping your confidence in proportion to the evidence.

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