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Must be decent at everything: the geometric mean, explained simply

The usual score lets a strength cover for a weakness. Sometimes that is right; for site selection it often is not. 'Must be decent at everything' (the geometric mean, the way the UN Human Development Index works) scores a place by its weakest link. Here is what changes on a London map.

Formula seriesBeginner friendlyLondon example
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ByTinkal Gogoi(LinkedIn profile, opens in a new tab)August 31, 20267 min read

Enormous demand, and no way to get there

Give a score two factors at equal weight: demand and accessibility. Area A has the best demand in the city and no transit at all. Area B is decent at both. The usual score adds the two up - and gives A and B exactly the same number.

For open-ended exploring that is fine: a strength makes up for a weakness. For choosing a site it is usually wrong, because a place nobody can reach is not half as good as a reachable one - it is out. Must be decent at everything is the other way to combine factors. It multiplies instead of adding, so a place is only ever as good as its weakest factor. The field calls it the geometric mean, and it is how the UN Human Development Index has worked since 2010 - deliberately, so a strong economy can no longer hide a failing school system.

Must be decent at everythingthe field's term: Geometric mean

Cafés within a walk, residents and transit stops within a walk, combined by 'Must be decent at everything'. Bright means solid on all three; dark means at least one is weak, however strong the others.

The same map, added up instead

Here is the identical set of factors under Balanced, the default, which adds them up with their weights. Compare the two: the balanced map is generous, with middling green nearly everywhere; the weakest-link map is decisive, keeping the places that pass on every count and dropping the rest.

Balanced: strengths make up for weaknesses, so most of the city scores somewhere in the middle.
Must be decent at everything: only the areas solid on all three factors stay bright.

Switching it on

It is one card at the bottom of the Score view's Advanced panel, after the factors it acts on. The description under the dropdown says the consequence out loud, because you should know it before you choose it.

Where to click

  1. Map type
  2. Score
  3. Advanced
  4. How the factors combine
  5. Must be decent at everything
The control, with its consequence in plain words: 'Somewhere with none of any one of them scores zero, however strong the rest are.'
The default, Balanced, with its own description. Flip between the two and the whole map re-scores.

One area, two verdicts

This is the whole idea in a single hover. The same London area, scored on cafés, residents and transit stops, each read in just its own spot. It has three cafés, 1 891 residents and no transit stop.

Balanced: 66 out of 100. The cafés and the residents carry it, and the missing transit costs it a third.
Must be decent at everything: 0 out of 100. No transit at all means the place fails, however good the rest is.

Zero is deliberate, and the control tells you

Multiplying by a factor measured at zero gives zero. The app does not soften that: if you say every factor is a must-have, an area with none of one of them is out. If that feels too strict for a factor - transit within a walk is usually the fairer question than transit in one small area - change that factor's Look at setting rather than the way the factors combine.

The maths, honestly

Each factor is first placed on a 0-1 scale (see how each factor is placed). Balanced then takes the weighted sum; Must be decent at everything takes the weighted product - equivalently, the average of the logs, which is why a zero is fatal: the log of zero does not exist, so the app treats a measured zero as a fail rather than as missing data.

Two areas, two factors at equal weight
Area A - demand 1.0, accessibility 0.0, Balanced
(1.0 + 0.0) ÷ 2 = 0.50
Area A - Must be decent at everything
√(1.0 × 0.0) = 0.00
Area B - demand 0.5, accessibility 0.5, Balanced
(0.5 + 0.5) ÷ 2 = 0.50
Area B - Must be decent at everything
√(0.5 × 0.5) = 0.50

Balanced cannot tell A from B. The geometric mean can, and it prefers the place that is decent at both - which is nearly always what site selection wants.

Use Balanced when factors trade off

A bit less footfall for a lot less competition is a real trade, and adding up is the honest way to express it. Also the right default for exploring.

Use Must be decent at everything for must-haves

When each factor is a requirement rather than a preference, multiply. Excellent-on-one, poor-on-another places drop; solid-on-all places rise.

Open this score of London

The whole idea, in four lines

  • Balanced adds the factors up, so a strength covers a weakness. Must be decent at everything multiplies them, so the weakest factor rules.
  • A factor measured at zero floors the score to zero - deliberately, and the control says so before you pick it.
  • It is the UN Human Development Index method: one strong dimension can no longer hide a failing one.
  • The map turns from generous and middling to decisive: fewer, sharper bright areas.
  • Hover the same area under both settings - 66 out of 100 against 0 - and you have the entire lesson.

Quick questions

What is the difference between 'Balanced' and 'Must be decent at everything'?

'Balanced' adds the factors up with their weights, so a strength can make up for a weakness. 'Must be decent at everything' multiplies them instead, so a place is only as good as its weakest factor.

Why does an area score zero?

Because it has none of one of the factors. Multiplying by zero gives zero, deliberately: if there is no transit at all, no amount of demand rescues the place. The control says so before you choose it.

Is this the same as the Human Development Index?

Yes in method. The UN's HDI switched to a geometric mean so that a strong economy could no longer hide a failing health or education dimension. The same logic applies to a location.

When should I use 'Balanced'?

For open-ended exploring, or when your factors genuinely trade off - a bit less footfall for a lot less competition, say. Use the weakest-link option when every factor is a must-have.

Does it change which areas rank highest?

Often, yes. Areas that are excellent on one factor and poor on another drop; areas that are solid on everything rise. Flip between the two and watch which places move - that is the story.

See it on a real map

The best way to understand the score is to play with it. The full app is on a free tier, with no sales call.

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About the author

Tinkal Gogoi(LinkedIn profile, opens in a new tab)Founder & Location Data Engineer

Builds MapBees's location-intelligence pipeline end to end — census and Overture Maps ingestion, H3 hex scoring, and the map interface — and writes the methodology behind every number on the site.