A great deal of information is released into the world under the label of anonymous. A name is removed, and what remains is presented as describing no one in particular. The label is reassuring. It is also, very often, mistaken.

The difficulty is that a person is identifiable not only by their name but by the particular combination of facts that is true of them. A date of birth is shared by many. A date of birth together with a place of residence is shared by far fewer. Add one or two further details, the kind that seem entirely ordinary, and the combination frequently describes a single individual and no one else.

This has been shown, repeatedly and rigorously, by researchers who have taken supposedly anonymous information and matched it back to named people with very little additional effort. The finding is consistent. It does not take much to turn a record of no one in particular into a record of a specific person.

For most people the practical risk of this is modest, because their pattern of life is unremarkable and blends into millions of others. For a person whose circumstances are distinctive, the opposite is true. An unusual residence, an unusual set of movements, an unusual profession or holding makes the combination of facts rarer, and a rare combination is easier, not harder, to resolve to a single name.

The lesson is not that the label of anonymous is always false. It is that it should not be relied upon. Information about a person shared as anonymous can still, under the right conditions, point back to them, and a careful assessment of exposure has to account for that.

The question, then, is never only what has a person's name attached to it. It is also what describes them clearly enough that a name is no longer needed. That is a harder question, and a more honest one.