The most useful single analytical shortcut in investment analysis is asking about base rates — the historical frequency of specific outcomes across broadly similar situations. Most retail investment commentary systematically neglects base rates in favour of specific-situation reasoning, which produces predictable errors when the specific situation turns out to be more typical than the reasoning acknowledged.
What base rate reasoning means
For any specific investment question, there is usually a broadly relevant historical dataset that provides some information about the frequency of specific outcomes. What percentage of stocks that traded at price-to-earnings multiples above 40 have produced positive returns over the following five years? What percentage of small companies that IPOed with negative earnings have outperformed the index over their first decade as public companies? What percentage of stocks that dropped 50% from recent highs eventually recovered to those highs?
Each of these questions has an approximate empirical answer that can be computed from historical data. That answer is the base rate for the outcome in question. Any specific analysis should acknowledge and account for the base rate rather than proceeding as if the specific situation is independent of historical patterns.
The systematic neglect of base rates
Kahneman and Tversky documented that humans systematically underweight base rates in favour of specific case features when making probability judgments. This is called base rate neglect and is one of the most-consistently documented biases in cognitive science.
The mechanism: specific case features (a company's compelling business narrative, a specific management team's vision, a particular market opportunity) feel more informative than statistical patterns across many broadly similar situations. The specific features are vivid and psychologically salient; the statistical patterns are abstract and less compelling.
The result: individual investment decisions typically overweight the specific case features and underweight the base rates. Every specific case feels like an exception to the historical pattern. In aggregate across many decisions, most cases turn out to be broadly consistent with historical patterns despite the individual reasoning that positioned each as an exception.
The IPO example
A specific example illustrates the pattern. Historical base rates on IPOs suggest that approximately half of newly-public companies underperform the broader market over their first five years, and roughly a quarter of them lose more than 50% of their value from the IPO price. This is the base rate.
When any specific IPO is being analysed, the specific case features (management team, market opportunity, competitive position, financials) can produce a specific case for why this particular company will outperform. The specific case may even be well-analysed. But if the specific reasoning does not acknowledge that this company is being evaluated against a base rate where half of similar companies have historically underperformed, the analysis is missing an important input.
The rigorous approach is to explicitly acknowledge the base rate and to require the specific case features to be strong enough to overcome it. A company that looks moderately attractive against a 50% base failure rate is much less compelling than the same specific analysis would suggest without the base rate context.
The base rate as sanity check
Base rate reasoning is particularly valuable as a sanity check on optimistic specific-case analyses.
If a specific analysis produces a projected return that would be in the top 5% of historical outcomes for broadly similar situations, the specific case features must be extraordinary to justify the projection. Most specific cases that produce such projections are not actually extraordinary; they represent optimistic analytical framing rather than genuinely rare underlying conditions.
If a specific analysis produces a projected outcome that would be in the middle of the historical distribution, the projection is more credible. It aligns with what historically happens in broadly similar situations.
If the specific analysis produces a projected outcome that would be in the bottom of the historical distribution (bearish projection), the analysis needs to acknowledge why this specific case is worse than average — and to accept that such projections are often too pessimistic (specific bearish cases often turn out closer to historical patterns than the specific analysis suggested).
Where base rates are hard to compute
Not every investment situation has an obvious base rate. Some genuinely novel situations do not fit historical categories cleanly. Some quantitative categories (specific sub-industries, specific corporate situations) may have inadequate historical data.
The corrective is not to abandon base rate reasoning but to acknowledge its uncertainty. If the base rate cannot be computed precisely, an approximate base rate from the most similar available category is still more useful than proceeding without any base rate anchor at all. The uncertainty in the base rate itself should be an input to the analysis.
The industry-specific base rates
Multiple base rates are worth carrying as general reference points for equity investing.
Approximately half of all public companies underperform the broader market over any given decade. This is definitional (half must underperform by construction) but the specific distribution matters — the underperformance is not symmetric; the median company underperforms by more than the median outperformer outperforms.
The base rate for individual stocks producing 10x returns over any decade is very low — typically single-digit percentage of the universe. Most 10x projections in retail commentary substantially exceed the historical base rate for such outcomes.
The base rate for companies with declining revenue to reverse to secular growth is low. Historical corporate turnaround success rates are modest, particularly for companies whose decline is driven by durable competitive shifts rather than temporary factors.
The base rate for high-multiple growth stocks to sustain multiple expansion is loose but generally lower than optimistic-case projections suggest. Multiple compression is more common historical outcome for high-multiple stocks than continued multiple expansion.
None of these base rates is a decision rule. Each is a contextual anchor that specific analyses should acknowledge and either accept or explicitly override with strong reasoning.
The specific practice
Three specific practices help incorporate base rate thinking into investment analysis.
Before analysing any specific case, ask what the broader category of similar cases has historically produced. This produces a specific anchor point against which the specific analysis can be compared.
When the specific analysis produces conclusions substantially different from the base rate, explicitly explain what specific features justify the divergence. If the divergence cannot be justified by specific features that materially distinguish this case, the analysis is likely too optimistic (or too pessimistic).
Track your own analytical predictions against outcomes over time. Most retail investors substantially overestimate their analytical accuracy; tracking against actual outcomes over long periods provides calibration that abstract self-assessment does not.
The rule to internalise
Base rate reasoning is one of the most consistently under-used analytical practices in retail investment analysis. Explicitly acknowledging historical base rates for the type of situation being analysed provides a specific anchor that produces sharper conclusions than analysis proceeding without that anchor. The practice is not difficult; it requires only the discipline to consistently ask what the historical pattern for broadly similar situations has been before proceeding to specific-case reasoning.
Educational content only. Not investment advice.