How confidence is rated

The stated range is back-tested against more than 35,000 recent sales — four times in five the actual price landed inside a range that wide. That claim is made only where it was measured: on same-block and same-development estimates whose floor area resembles the sales behind them, and on a smaller measurement of same-street ones. Town-wide estimates are the part of this model we have been least able to test — only 14 could be back-tested at all — and any lookup where the recorded sales do not describe your property is outside the measurement too. In those cases the page says so, names the reason, and widens the range as a judgement rather than a measurement.

What moves the rating is what was measured to move the error, and nothing else. High means the sales behind the estimate are recent (within two years for HDB, one year for private) and closely priced — and, for a condo or EC, that there are at least ten of them; the range is ±7%. Medium means one of those does not hold, or your floor area is more than 10% off the typical sale; the range is ±10%, widened to ±12% when the size gap passes 20%, to ±14% for a condo or EC more than 35% off, and to ±15% for a same-street estimate. How many sales there are does not count for HDB: it was measured to make no difference there. Low means the sales do not describe your property well enough to measure — an HDB flat more than 35% off the typical size, a size, storey or lease outside their range, town-wide sales, or no floor area given.

When the comparables do not actually describe your property — a floor area or storey well outside the range they cover — the page says so and lowers its confidence, or declines to answer. That is deliberate: a confident number from unrelated sales is worse than no number.