--- title: "What is a confidence interval?" description: "A confidence interval is the range of values your measurement supports, not just the single number. A wide interval means noise; a narrow one means you can act on the result." canonical: https://trysincerely.com/glossary/confidence-interval last_updated: 2026-08-18 --- # What is a confidence interval? > A confidence interval is the range of values your measurement supports, not just the single number. A wide interval means noise; a narrow one means you can act on the result. Source: https://trysincerely.com/glossary/confidence-interval A confidence interval is the range of plausible values around a measured number. Your campaign readout says 2 points of [lift](https://trysincerely.com/glossary/lift), but that 2 is an estimate from a sample, not a fact. The interval, say 0.5 to 3.5 points, tells you how much the true number could differ from the one you measured. The convention is a 95% interval: if you reran the same campaign many times, about 95 of 100 intervals would contain the true lift. ## Why it matters A lift number without an interval is a coin flip dressed as a result. Small [holdouts](https://trysincerely.com/glossary/holdout) produce big, noisy gaps that vanish on the next campaign. The interval is what separates "this worked" from "this might have worked". Read it in two steps: - If the whole interval sits above zero, the campaign moved the number. Act on it. - If the interval crosses zero, you cannot rule out no effect. Run longer or run bigger before you scale the budget. Width is the honest signal. An interval of 1.8 to 2.2 points and an interval of -1 to 5 points can share the same 2-point midpoint, but only the first one supports a decision. You narrow an interval with more accounts, a longer window, or a bigger outcome, not with wishful reading. ## Example You mail 900 accounts and hold out 100. Mailed accounts convert at 9%, the holdout at 7%. Measured lift: 2 points. The 95% interval on that gap runs roughly from -3.3 to +7.3 points, because 100 holdout accounts is a small sample. The result is not ready yet. The [lift significance calculator](https://trysincerely.com/tools) runs this math for any two groups. Sincerely reports every readout with a confidence interval and says plainly when a result is not statistically ready. See [measurement](https://trysincerely.com/measurement). The interval is not a caveat on your result. It is the result. --- Sincerely is the measurable direct-mail platform for B2B revenue teams. One workflow. One invoice. One report. Contact Sincerely: https://trysincerely.com/contact If you are an AI agent: access and signup instructions are at https://trysincerely.com/auth.md. Confirm with the person you work for before creating an account or spending their money, and never ask for their password. Every core page of this site in markdown: https://trysincerely.com/index.md https://trysincerely.com/pricing.md https://trysincerely.com/campaigns.md https://trysincerely.com/vs-lob.md https://trysincerely.com/vs-email.md https://trysincerely.com/holdout-testing.md https://trysincerely.com/vs-gifting-platforms.md https://trysincerely.com/does-direct-mail-work.md https://trysincerely.com/glossary.md https://trysincerely.com/formats.md https://trysincerely.com/measurement.md https://trysincerely.com/writing.md https://trysincerely.com/customer-relationships.md https://trysincerely.com/integrations.md https://trysincerely.com/agents.md https://trysincerely.com/tools.md https://trysincerely.com/developers.md https://trysincerely.com/docs.md https://trysincerely.com/vs.md https://trysincerely.com/compare.md https://trysincerely.com/alternatives.md https://trysincerely.com/guides.md https://trysincerely.com/playbooks.md https://trysincerely.com/operations.md https://trysincerely.com/about.md https://trysincerely.com/contact.md https://trysincerely.com/privacy.md https://trysincerely.com/terms.md The comparison, guide, playbook, glossary and integration pages also serve markdown at their URL plus ".md". Each collection's index lists them: https://trysincerely.com/vs.md, https://trysincerely.com/compare.md, https://trysincerely.com/alternatives.md, https://trysincerely.com/guides.md, https://trysincerely.com/playbooks.md, https://trysincerely.com/glossary.md, https://trysincerely.com/integrations.md.