Reading the numbers

Reported crime counts what somebody told the police and the police wrote down. Two towns with identical experiences can post very different rates if one has a department that records diligently and the other does not, or if residents in one are likelier to call at all.

The larger problem is participation. Departments submit returns to the FBI voluntarily, and many submit for part of a year or not at all. A rate built on nine months of reporting, annualized, looks like a real number and is not one. That is why nothing appears here unless a department reported all twelve months — a rule that withheld 1,578 department-years.

Watch the denominator too. A city that draws a large daytime workforce is policing far more people than live there, and every incident involving a commuter or a visitor lands in a rate divided by the residential population.

What it does answer: how much crime is being recorded here, relative to places recording theirs the same way.

A death certificate is filed whether or not the local police department submitted anything that year, which makes homicide the most consistently counted violent outcome in the country. When a crime rate and a homicide rate disagree sharply, the crime rate is usually the one bending.

The trade is resolution. These are published by county and nowhere finer, and a county can hold a dense city center and a quiet suburb at once. Fulton County records 13.1 homicide deaths per 100,000 and contains both downtown Atlanta and Johns Creek — a figure that describes neither. That is the whole reason it is shown here and never folded into a score.

Every state writes its own test and sets its own passing mark. A state that sets a demanding bar produces lower proficiency rates than one that does not, and the difference between them measures the exam rather than the schools. Comparing 62% in Georgia against 62% in Massachusetts is comparing two unrelated things.

So proficiency is ranked within its own state here. That is the only comparison the data supports, and it is also the comparison a family actually faces — they are choosing between schools in one state, not across fifty.

Two smaller traps, both of which changed figures on this site. Fully virtual schools enroll students from across a state while reporting one administrative address; counting them gives a town a student body it does not have. And results for small groups are published as a range rather than a number — a school reported as “50 to 100 percent” arrives as its midpoint, 75, which is not a measurement of anything. One Oregon town read as the best in the state on six tested fourth-graders before that was caught.

What it does not answer: what a school adds. Proficiency tracks the circumstances students arrive with at least as strongly as it tracks teaching, which is why free-lunch eligibility is shown beside it and never scored.

The census figure is what tenants are currently paying, averaged across everyone in place — including households that signed a lease a decade ago and have never moved. It is an accurate description of what the existing tenancy stock costs.

Fair market rent is HUD’s estimate of the 40th percentile of what recent moverspay. It is an administrative standard, set for housing assistance programs rather than surveyed from listings, and it is the closest published figure to what signing a lease today costs. In Palo Alto the two differ by twenty percent; in Johns Creek by fourteen.

If you are moving, the second number is the one that will land on you, and the gap between them tells you how fast the market has moved away from people already in place.

The index compares what a dollar buys against a national average of 100. It is weighted the way the country as a whole consumes, not the way you do, and the spread between places is driven almost entirely by housing — groceries and goods vary far less than the headline suggests.

It also excludes taxes, which frequently run in the opposite direction. A state with a low price level and a high income tax may leave you no better off, and the index will not tell you that.

Used properly it is a conversion rate: divide a salary by it to compare offers in different cities in the same currency. Used improperly it is a claim about your monthly outgoings, which it is not.

This is the question that catches people most often, because the answer is rarely stated. A figure printed on a page about a city may have been measured for the county around it, the metropolitan area, or the city itself, and the three can differ enormously.

The rule worth carrying: the wider the geography, the more it averages away exactly the difference you are trying to see. County crime tells you little about which suburb to pick. Metropolitan price levels are genuinely regional, because people shop and commute across a metro, so there the wider figure is the right one. Every measure here states which geography it was measured on, and inherited figures say so explicitly.

Where a figure is missing here, it is missing for a reason that is stated: no department reports for this town, no air monitor sits in this county, too few students sat the test. That is more honest than an estimate, and more useful — a town with no municipal police department is telling you something real about itself.

Be wary of any source that has a number for everywhere. Complete coverage of a patchy federal record means somebody filled the gaps, and filling gaps is where confident-sounding fiction comes from.