The fourth criterion of credibility is expertise.

This relates to whether an observer has the necessary background knowledge and understanding to correctly interpret the evidence before them.

It is sometimes difficult to accurately interpret evidence without specialised knowledge. For example, someone watching a cricket match might not fully appreciate the tactical battle between bowler and batsmen, and as a result might reach a false conclusion as to how well each is playing. They may think that the batsman is playing poorly because he isn’t scoring any runs, for example, when actually he is playing excellent defensive strokes at a time when defence is the priority.

Similarly, if a person who doesn’t know how to drive were trying to apportion blame regarded a road accident that they had witnesses, then they may not be able to accurately judge who was at fault. Deciding whether a driver had taken a corner too quickly, or whose right of way it was at a junction, involves a certain amount of expertise, which a non-driver may well lack.

Possessing relevant expertise thus strengthens a witness’s credibility; lack of relevant expertise thus weakens it.