Generally, the opinion of a witness is not admissible unless the witness is an expert in the field on which the opinion is given. A doctor may be able to give an opinion on whether a particular blow may have caused the death of a person, while a non-medical witness cannot give such an opinion as evidence.
There are many rules and laws surrounding expert opinion evidence and what is allowed by the courts, in particular it has to be directly relevant to the facts in issue and the expert must have expert qualifications or experience in the relevant field.
Lay (non-expert) witnesses are able to give evidence on matters of fact, or on those things for which the experience of everyday life is sufficient, such as the weather and general identity.
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