Class and individual evidence: how specific is a clue?

Not every clue points at one person. A footprint made by a common trainer could have been made by thousands of people. A footprint with a rare cut in its sole, that you find again on a suspect's shoe, points much more strongly. Forensic scientists sort clues by how specific they are.

Class characteristics are shared by a whole group of items or people.

• A shoe's size and tread pattern (every pair of that model has them).
• A blue cotton fibre (millions of garments contain it).
• The ink formula of a popular pen.

Individual characteristics are specific to one source, as far as anyone knows.

• A cut or wear pattern on one particular sole, picked up by chance as it was used.
• A torn piece of paper that fits the torn edge of another, like a jigsaw.
• A full DNA profile (very specific, though not for identical twins).
• A fingerprint, which is treated as individual in everyday practice (World 2 looks at the limits of that).

It is a scale, not two boxes. Ask: how many sources in the world could have left this trace? If a thousand people share it, it is class evidence. If you find a trace that is rarer and then another that is also rare and does not depend on the first (a shoe size, plus a fibre colour, plus a cut in the sole), together they narrow things much more. But be careful: two clues that come from the same cause (say, two fibres from the same scarf) do not count twice.

A match is not guilt. Suppose a trace is shared by 1 in 1,000 people. That sounds rare. But in a town of 20,000 people, about 20,000 / 1,000 = 20 people would match it. A match shows that a suspect cannot be ruled out, and it must be weighed against everything else. Always ask two different questions: how likely is this match if the suspect did it? and how likely if someone else did? They are not the same question, and mixing them up is a famous reasoning error.

This is also why excluding someone is usually stronger than matching: one clear mismatch (a 24 cm shoe cannot make a 29 cm print) can rule a person out, while a match only keeps them in the pool.