Opinion evidence refers to evidence of what the witness thinks, believes, or infers in regard to facts, as distinguished from personal knowledge of the facts themselves. In common law jurisdictions the general rule is that a witness is supposed to testify as to what was observed and not to give an opinion on what was observed. However, there are two exceptions to this rule: expert evidence and non-expert opinion given by laymen which people in their daily lives reach without conscious ratiocination.

General rule

In general, witnesses should testify only as to the facts observed and should not give opinion.

The main rationale for such a rule is that the admission of opinion evidence would not assist, or might even mislead, the court and in particular the jury. This is because opinion evidence is usually irrelevant. Moreover, admission of such evidence would usurp the functions of the jury, which alone should be the tribunal of fact and draw its own inferences.

Expert evidence

An expert witness is a witness

Witness

A witness is someone who has firsthand knowledge about an event, or in the criminal justice systems usually a crime, through his or her senses and can help certify important considerations about the crime or event. A witness who has seen the event first hand is known as an eyewitness...

, who by virtue of education

Education

Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...

, training

Training

The term training refers to the acquisition of knowledge, skills, and competencies as a result of the teaching of vocational or practical skills and knowledge that relate to specific useful competencies. It forms the core of apprenticeships and provides the backbone of content at institutes of...

, skill

Skill

A skill is the learned capacity to carry out pre-determined results often with the minimum outlay of time, energy, or both. Skills can often be divided into domain-general and domain-specific skills...

, or experience

Experience

Experience as a general concept comprises knowledge of or skill in or observation of some thing or some event gained through involvement in or exposure to that thing or event....

, is believed to have expertise and specialised knowledge

Knowledge

Knowledge is a familiarity with someone or something unknown, which can include information, facts, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject...

 in a particular subject beyond that of the average person, sufficient that others may officially and legally rely upon the witness's specialized (scientific

Scientific evidence (law)

This article is about the legal concept. For scientific evidence in pure science, see Scientific evidence.- Role :The educating witness teaches the fact-finder about the underlying scientific theory and instrument implementing theory...

, technical or other) opinion about an evidence

Evidence

Evidence in its broadest sense includes everything that is used to determine or demonstrate the truth of an assertion. Giving or procuring evidence is the process of using those things that are either presumed to be true, or were themselves proven via evidence, to demonstrate an assertion's truth...

 or fact

Fact

A fact is something that has really occurred or is actually the case. The usual test for a statement of fact is verifiability, that is whether it can be shown to correspond to experience. Standard reference works are often used to check facts...

 issue within the scope of his expertise, referred to as the expert opinion, as an assistance to the fact-finder. Expert witnesses may also deliver expert evidence about facts from the domain of their expertise. The facts upon which an expert opinion is based must be proved by admissible evidence. The duty of experts is to furnish the judge with the necessary scientific criteria for testing the accuracy of their conclusions, so that the judge or jury can form their own independent judgment by the application of these criteria to the facts proved.

Non-expert opinion

Non-expert opinion refers to the opinion given in restricted circumstances by laymen or persons who do not possess any expertise. These circumstances mainly concern matters of every day life where a person may be expected to give opinions and which opinions may be safely acted upon by others.

Such circumstances cannot be definitively laid out in a closed list of cases. However, cases in which non-expert opinion has been admitted include:

• Apparent age of a person
• Apparent age of objects
• Speed
• Weather
• Identification of handwriting
• Eyewitness identification
  Eyewitness identification
  Eyewitness identification, in criminal law, is evidence received from a witness "who has actually seen an event and can so testify in court"....
  
  
• Identification of physical objects
• The general body condition or emotional state of a person
• The general condition of objects
• The approximate value of objects
• Approximate distance
• Time
• Ability to speak and understand a language