Football fans who abuse linesmen over their offside decisions appear to have their case validated by science. According to Dutch research released this week, linesmen make a mistake in one out of every five offside decisions.

The psychologists at Amsterdam's Free University believe that linesmen are usually in the wrong position to judge whether a player is offside - and are therefore vulnerable to an optical illusion. The researchers found that linesmen are usually behind the position of the defender when they have to make the judgment call.

By a trick of perspective, an attacking player who goes outside the defender looks as if he is onside. But an attacker who goes inside the defender in the same position and from the same viewpoint, looks offside. The reverse is the case if the situation occurs on the other side of the field from the linesman. There, the player who goes outside the defender is more likely to look offside to the man with the flag. In other words, an attacker could be ruled offside less often if he runs inside or outside the defender, depending on the position of the linesman.

The researchers' conclusion is based on exhaustive work. They got together two elite youth football teams and generated potential offside positions which were judged by professional linesmen. A video camera kept record. In 200 offside situations, the linesman made 40 errors, the researchers found. Their conclusions were backed by analysis of offside decisions made in the 1998 World Cup.