What Is Base Rate?

Base rate is the prior probability of an event in a broad reference class before adjusting for case-specific evidence. In venture, it grounds fundraising timelines, exit expectations, and market sizing in historical reality.

How it works

If historical data suggests only a minority of seed companies raise a Series A within twenty-four months, that minority figure is the base rate for “seed → A” transitions. A founder with strong metrics updates the probability upward; a founder with no product updates downward — but neither starts from “we will definitely raise because the market is huge.”

Investors use base rates in underwriting: conversion from pilot to paid enterprise contracts, biotech Phase 2 success rates, or consumer app retention cohorts. Ignoring base rates leads to base rate neglect — overweighting vivid founder narratives and underweighting silent statistics.

Base rate does not mean your startup is average; it means default assumptions should reflect the group until dispositive data arrives. Founders who articulate why their cohort differs — faster growth, proprietary channel, regulatory tailwind — earn credit without ignoring silent statistics.

Why it matters

• Founders: Plan runway for multiple fundraising outcomes using realistic transition rates, not best-case closes.
• Investors: IC memos that cite base rates alongside upside cases read more disciplined to LPs.
• LPs: Over-allocation to venture without liquidity planning forces secondary sales or missed capital calls — base rate history suggests long J-curves before distributions.

Common mistake

Citing a massive TAM as proof you will win. Market size does not change the base rate of startup failure; execution and distribution do.

Related ideas

Reference class forecasting, power law, /glossary/bottom-up-tam, and outside view.