A powerful four-door saloon glides to a halt by the kerbside. Quickly two men leap from the nearside doors, one with a half-brick in his hand, both holding small sacks. A third man comes from the offside, a length of metal piping in his hand. The driver remains at the wheel of the car, his engine running. All wear hats, raincoats with the collars turned up, and scarves over the lower part of their faces.

Crash! As the brick goes through the jeweller's window, the first two run to the jagged hole, while the third covers their retreat. They know exactly what they want. In less than thirty seconds they are racing back to the car. From the shop comes a startled assistant. He lunges at the rearmost of the men and receives an elbow in the throat for his pains. As he falls, gasping for breath, he courageously snatches at the hat and scarf of the raider. Four passers-by have a momentary glimpse of the thief's face. Then he too is in the car, the engine races, gears slam and they are gone.

‘Did you see him?’—‘Whatever happened?’

‘What did he look like?’—‘Oh, that poor man from the shop. Do something for him, somebody.’—‘Darkhaired, wasn't he?’—‘ No, not dark exactly, sort of brown.’

‘Has anyone phoned for the police?’—‘He had a moustache, anyway.’—‘Don't be daft, he was cleanshaven.’—‘ About five foot nine, I would say.’—‘Never.Why, he wasn't as tall as my husband.’—‘What was he wearing?’—‘Quite a young chap, I thought. What did you think?’

Four people saw him, for a few seconds only; and four people will give four very different descriptions to the police. They will say what they saw; they will say what they thought they saw; and they will say what they heard others say they saw. Out of this garbled information the police will have to compile a description of the wanted man.

They will do it methodically. First the witnesses will separately give their accounts of what happened and their descriptions of the man they saw. By skilful questioning the officer will prompt each witness to a more accurate account of the man's features.