Methodology
Propkaki publishes four named metrics. Each one has a written definition, a stated formula, a declared scope, a suppression rule and a version history — so that a figure quoted from this site can be checked, reproduced, and dated. This page indexes all four and names the primary sources they are computed from.
The four metrics
The Propkaki Curve™
How much of an HDB flat’s resale price is explained by the lease it has left. Median resale PSF banded by remaining lease, so lease decay can be read as a price series rather than argued about.
The Propkaki Gap™
The percentage by which freehold private condominiums transact above 99-year leasehold ones, per square foot, island-wide. Computed from every URA caveat in the rolling five-year window.
The Propkaki Wave™
How many HDB flats reach the end of their five-year Minimum Occupation Period each quarter — the supply of flats that become legally sellable, ahead of them reaching the market.
The Propkaki Affordability Index™
Years of gross median household income needed to buy the median HDB resale flat. One number for whether housing is getting harder to afford, independent of whether prices alone went up.
What a Propkaki methodology page states
Every one of the four follows the same structure, so that two metrics can be compared on how they were built and not only on what they say:
| Section | What it commits to |
|---|---|
| Definition | The metric in one sentence, followed by the formula in full. |
| Scope and data source | Which records are included, which are excluded, and why each exclusion exists. |
| Computation steps | The numbered sequence from raw records to published series, reproducible by a third party. |
| Suppression rules | The minimum sample below which a period is withheld rather than published thin. |
| Interpretation notes | What the metric does not control for — stated by us, not left for a reader to discover. |
| Series keys and API access | The exact query that returns the published numbers, free and without a key. |
| Citation | The preferred citation string, including the methodology version. |
| Version history | Every change to the method, dated. A revised metric gets a new version, never a silent edit. |
Primary sources
Propkaki computes from official Singapore government data. The metrics are derived aggregates — the underlying records belong to the agencies below, and are linked here so any published figure can be traced back to them.
HDB Resale Flat Prices ↗
The full resale transaction record from 1990, published by HDB on data.gov.sg. Feeds the Curve, the Wave and the Affordability Index.
Urban Redevelopment Authority ↗
Private residential caveat records and the Private Residential Property Price Index. Feeds the Gap and every private-market tracker.
SingStat Table M810361 ↗
Median monthly household income from work, Department of Statistics Singapore. The denominator of the Affordability Index.
MAS — Total Debt Servicing Ratio ↗
The borrowing constraint that sits underneath any affordability reading. Referenced, not computed from.
The full question → official source → tracker → API mapping is on the data source map.
Citation formats, BibTeX and licence terms: /market/cite. Every citation should carry the methodology version, because a metric revised under a new version is a different measurement.
Revisions to any metric are recorded in that metric’s version history and in the tracker changelog.