A short-term test, 48 hours to 90 days under Ohio's protocol, is the tool for a home sale and a first screening. A long-term test of more than 90 days shows the year-round average and is the better basis for a borderline decision.
More detail
Ohio's recommended strategy for a home that is not being sold starts with a short-term test in the lowest livable level, then adds a second short-term test or a long-term test when the first result lands between 4 and 8 pCi/L. For a sale, only the short route is practical, and Ohio requires at least 48 hours using either two side-by-side devices or a continuous monitor. Long-term devices, such as alpha-track detectors, suit a homeowner who wants the whole seasonal swing of a zone 5A year, and they need far less babysitting than a closed-house short test. Neither type is more official than the other; they answer different questions. The comparison page lays them side by side.