# Methodology and sources

How BTO Radar's estimates are made, what they assume and where the inputs come from.

## How the BTO ballot works

HDB gives each applicant in a sales exercise a queue number by computerised ballot. Applicants with more ballot chances are more likely to get an earlier number. HDB then invites applicants to book a flat in queue order until the flats are taken. Because some invited applicants decline, HDB usually invites more people than there are flats.

## The model

A ballot weighted by ballot chances is equivalent to giving every applicant a random arrival time that comes sooner the more chances they have (an exponential distribution with rate equal to their chances), and ordering applicants by it. With many applicants, the people invited are those who arrive before a cut-off. We find the cut-off at which the expected number of invited applicants equals the number of invitations, and an applicant with *k* chances is invited with probability 1 − e^(−k·t).

Per flat, there are *rate* applicants and 1 ÷ (1 − *decline*) invitations. If invitations exceed applicants, everyone is invited.

When everyone has the same number of chances, the estimate reduces to 1 ÷ (rate × (1 − decline)).

## What it leaves out

- Category quotas (for example the share of flats set aside for first-timers) and priority schemes such as the Married Child Priority Scheme or the Family Care Scheme.
- Flat-type, block and unit preferences: being invited does not guarantee the unit you want.
- Changes to ballot rules after the date shown on this page.

Use the application rate HDB publishes for your own category where it exists; it already reflects much of the quota effect.

## Ballot chances

- First-timer applicants generally get two ballot chances and second-timers one ([MND](https://mnd.gov.sg/newsroom/speeches/view/written-answer-by-ministry-of-national-development-on-ballot-chances-for-first-timer-bto-flat-applicants)).
- First-timer families who have been unsuccessful in earlier non-mature estate applications may receive additional chances ([MND](https://mnd.gov.sg/newsroom/speeches/view/written-answer-by-ministry-of-national-development-on-additional-balloting-chances-for-unsuccessful-first-timer-families-bto-application-in-non-matured-estates)).
- The number HDB shows on your own application is the one to use.

## Decline rate

HDB has said that a large share of invited applicants decline to book; one report cites about 40% ([Home & Decor](https://www.homeanddecor.com.sg/property/first-timers-have-good-chance-to-book-bto-flat-as-40-of-applicants-invited-to-do-so-decline-says-hdb)). The rate varies by project and exercise, so the calculator lets you choose it and shows the no-decline case for comparison.

## Data sources

- Launches, projects and application rates: collected automatically several times a day from HDB's [Flat Portal](https://homes.hdb.gov.sg/). Each run is checked before anything is published: the response must have the expected structure, every number must be in a plausible range, and a figure already published that HDB later reports differently is held for 24 hours before it is updated. If a check fails, nothing from that run is published.
- Corrections: email hello@btoradar.com. Material corrections are noted on the page concerned.