Neighborhood data glossary

Plain-English definitions for the terms that turn up when you research a neighborhood. Most come from government datasets and are written for statisticians rather than for people trying to move house.

Areas and boundaries

Census block group

A small area, usually 600 to 3,000 people — roughly a neighborhood, or a few streets in a dense city. It is the smallest area the Census publishes most statistics for, which makes it the finest grain at which you can honestly compare things like crime or income.

Census tract

A larger area of about 1,200 to 8,000 people, made up of several block groups. Tracts are more stable over time, so they are used where a block group would be too small to give a reliable number.

Metro area

A city plus the surrounding places economically tied to it — the commuter belt, essentially. It is why population figures for a “city” vary so much between sources: some count the city limits, some count the metro.

School attendance zone

The area whose children are assigned to a particular school. These do not follow neighborhood names or postal boundaries, and they change. Two houses on the same street can be zoned to different schools.

Parcel

One piece of land as the county records it, with its own boundary and tax record. The unit behind property tax and ownership data.

Getting around

Drive-time area (isochrone)

Everywhere you can reach from a point within a set time. Drawn on a map it is never a circle — roads, rivers and hills stretch it in some directions and cut it off in others. It is the honest version of “within 20 minutes,” and you can draw one on the commute map.

Headway

How long you wait between buses or trains. A ten-minute headway means turning up without checking a timetable; a forty-minute one means planning your day around it. Often more important than distance to the stop.

Transit frequency

The same idea from the service’s side — how many departures per hour. Frequency, not proximity, is usually what decides whether someone can live without a car.

Risk and hazard

Flood zone

A FEMA designation for land expected to flood from a river, lake or the sea. It does not cover rainfall that overwhelms drains, which is why properties outside any zone still flood. More on how flood zones work.

Zone AE, Zone X, Zone VE

The common flood codes. AE is high risk with a known expected water level. VE is high risk on a coast, with waves on top. X is outside the high-risk area — lower risk, not no risk.

Wildfire hazard potential

How likely land is to burn, based on vegetation, dryness and terrain. It describes the landscape, not your house, and says nothing about whether a fire will actually start. See how we show wildfire risk.

Impervious surface

Ground that water cannot soak into — roofs, roads, car parks. The share of an area that is impervious drives how fast rain becomes runoff, which is why building up an area can worsen flooding well beyond the new buildings.

Crime statistics

UCR and NIBRS

The two FBI crime reporting systems. UCR was a simple count of the most serious offence in an incident. NIBRS, which replaced it, records every offence and much more detail. The changeover left gaps in the national data, because not every department switched at the same time.

Part I offences

The older UCR grouping of serious crimes — homicide, robbery, burglary and so on — still widely used as shorthand for “serious crime” in comparisons.

Per capita rate

A count divided by population, usually expressed per 1,000 or 100,000 people. It is what makes two places comparable: a busy downtown will always have more incidents than a quiet block simply because more people are there.

Reported crime

Crime somebody told the police about. Every crime statistic is really a statistic about reporting, and reporting rates differ between places and between offence types. Worth reading why crime maps disagree.

Costs and value

Median vs average

The median is the middle value; the average is the total divided by the count. A few very expensive properties pull an average upward and leave a number that describes nothing real, which is why area prices are usually quoted as medians.

Comparable sale (“comp”)

A recent sale of a similar nearby property, used to judge what something is worth. Comps are what agents and appraisers argue about, because whether two properties are genuinely similar is a judgement call.

Assessed value

The value a local authority puts on a property for tax purposes. It is often well below what the property would sell for, and it updates on its own schedule, so it is a poor guide to market price.

Effective property tax rate

Annual tax as a share of the property’s value. The number worth comparing between areas, because headline rates are applied to different assessment bases and are not comparable directly. See tax rates on the map.

Days on market

How long a property has been listed. High numbers across an area suggest a slower market and more room to negotiate; very low numbers suggest the opposite.

Growing things

Hardiness zone

A number describing the average coldest night of the year, used to work out which plants survive winter outdoors. More on planting zones.

Last frost date

The typical date of the final spring frost. It governs when it is safe to plant out, and is a different question from which plants survive the winter.

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