A decision-maker is not obliged to listen to anybody's opinion, and the general rule is that witnesses give facts. Expert evidence is the exception, and it is a narrow one: the field has to be a real one, the person has to be qualified in it, the opinion has to rest on a basis that can be examined, and the report has to say what that basis is. This subject sets out how an opinion qualifies, what a report must contain, and what happens when two qualified people reach opposite conclusions.
An opinion rests on material, assumptions and a method, and each of the three is a point at which it can fail. Material may be incomplete or chosen by somebody with an interest in the result, assumptions may be contested facts the decision-maker has yet to resolve, and a method may be sound in general but wrong for this particular question. Attacking the conclusion directly almost never works.
