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4.14.1 - General Principles of Opinion Evidence

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What is Opinion Evidence?

  1. The "opinion rule" provides that "evidence of an opinion" is generally inadmissible (Evidence Act 2008 s76).[1]
  2. While the Evidence Act 2008 does not define the term "opinion", it has been held that it refers to an "inference drawn from observed and communicable data" (Allstate Life Insurance Co v ANZ Banking Group Ltd (No 5) (1996) 64 FCR 73) or "evidence of a conclusion, usually judgmental or debatable, reasoned from facts" (R. W. Miller v Krupp Australia Pty Ltd (1992) 34 NSWLR 129. See also Hodgson v Amcor Ltd [2011] VSC 272).
  3. Evidence of an "opinion" can be distinguished from evidence of a "fact". Where a witness simply gives evidence of something he or she observed, or of a particular state of past or present affairs, that will not be opinion evidence (see, e.g., Bodney v Bennell (2008) 167 FCR 84; Australian Securities and Investments Commission v Vines [2003] NSWSC 1095; Hodgson v Amcor Ltd [2011] VSC 272).
  4. Evidence of an "opinion" can also be distinguished from evidence of "experience". Evidence of a witness’ experiences will not be opinion evidence, unless the witness proceeds to draw some inference from those experiences (see, e.g., Clark v Ryan (1960) 103 CLR 486).[2]
  5. Hearsay evidence of an opinion is itself opinion evidence (R v Whyte [2006] NSWCCA 75; Jackson v Lithgow City Council [2008] NSWCA 312. But cf Australian Securities and Investments Commission v Rich (2005) 216 ALR 320).
  6. The following matters have been held not to be opinion evidence for the purposes of the Uniform Evidence Acts:

    Identification Evidence

  7. It appears that identification evidence may be either evidence of fact or evidence of opinion, depending on the circumstances:
  8. This means that, in every case involving identification evidence, the trial judge must examine the nature of the evidence proposed to be adduced, and all of the relevant circumstances, to determine whether the evidence is opinion evidence (and is thus subject to the admissibility provisions outlined below) (R v Drollett [2005] NSWCCA 356).

    Evidence in the Form of an Opinion

  9. The opinion rule refers to "evidence of an opinion". It has been suggested that this phrase covers both of the following:

    Admissibility of Opinion Evidence

  10. The opinion rule states that evidence of an opinion is not admissible to prove the subject matter of the opinion (Evidence Act 2008 s76).[5]
  11. There are two main exceptions to the opinion rule:
  12. Other exceptions to the opinion rule include:

    Evidence Must Be Relevant

  13. Opinion evidence, like any other evidence, is only admissible if it is relevant (Evidence Act 2008 ss55-56).
  14. The relevance of opinion evidence may depend upon an assessment of its factual basis. Opinion evidence will not be relevant if it does not have a rational factual basis (Evidence Act 2008 s55; R v Panetta (1997) 26 MVR 332).

    Abolition of Common Knowledge and Ultimate Issue Rules

  15. Opinion evidence may not be excluded simply on the basis that it is about:
  16. Although the effect of this provision is to abolish the common knowledge and ultimate issue rules, it should be noted that s80 does not make any evidence admissible. It simply provides that opinion evidence is not inadmissible on the specified bases (Idoport Pty Ltd v National Australia Bank Ltd (2000) 50 NSWLR 640).
  17. This means that evidence which is about a matter of common knowledge, or which relates to the ultimate issue in a proceeding, should not always be admitted. Such evidence may be inadmissible on another basis (e.g., because it is not wholly or substantially based on specialised knowledge, and thus does not comply with the requirements of s79), or it may be excluded at the judge’s discretion under ss135 or 137 (see, e.g., R v GK (2001) 53 NSWLR 317; Idoport Pty Ltd v National Australia Bank Ltd (2000) 50 NSWLR 640; Yates Property Corp v Boland (1998) 85 FCR 84).

    Expert Evidence in Homicide Cases Involving Family Violence

  18. In homicide cases involving family violence, evidence may be led to show:

    Identification of Factual Basis

  19. At common law, when expert opinion evidence is admitted:
  20. This allows the jury to determine whether the opinion has any value in light of their findings of fact, and to assess the weight to be given to that opinion (Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313; R v GK (2001) 53 NSWLR 317).
  21. While the Evidence Act 2008 does not require witnesses to state the factual basis for their conclusions or to demonstrate their reasoning process, in the other Uniform Evidence Act jurisdictions they have generally been required to do so (see, e.g., Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705; Hevi Lift (PNG) Ltd v Etherington [2005] NSWCA 42; Seven Network Ltd v News Ltd (No 15) [2006] FCA 515).
  22. Thus, it may remain prudent to require expert witnesses to identify the facts they have relied upon to form their opinion (see, e.g., Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313; R v GK (2001) 53 NSWLR 317; Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705. See also R v Anderson (2000) 1 VR 1; R v Ryan [2002] VSCA 176; R v Johnson (1994) 75 A Crim R 522).
  23. Experts should also be required to explain how their opinion was reached, and the application of their expertise (R v Johnson (1994) 75 A Crim R 522; R v Haidley & Alford (1984) VR 229).
  24. Experts may consider matters of common knowledge, as well as their specialised knowledge, when forming their opinion. They should explain that they are relying on this accumulation of knowledge, as well as the way in which they acquired that knowledge (Velevski v R (2002) 187 ALR 233; Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313).
  25. The need to identify and explain the factual basis of an expert opinion will only arise where that basis is contested. If evidence is adduced without objection, the trial judge may ordinarily assume that all matters crucial to the admissibility of the evidence are conceded by the opposing party (Sydneywide Distributors Pty Ltd v Red Bull Australia Pty Ltd (2002) 234 FCR 549; [2002] FCAFC 157).
  26. While the failure of an expert to explain his or her reasoning process will not necessarily render the evidence inadmissible, it will usually mean that it is less persuasive for the jury (HG v R (1999) 197 CLR 414 per Gaudron J. See also Guide Dog Owners’ & Friends’ Association Inc v Guide Dog Association (NSW & ACT) (1998) 154 ALR 527).

    Hearsay Evidence Admitted as the Basis of an Opinion

  27. Witnesses’ opinions will often be based on out-of-court representations. Evidence of those representations will generally be admissible to explain the assumptions on which an opinion is based (see, e.g., R v Lawson [2000] NSWCCA 214).[8]
  28. If admitted for this purpose, an out-of-court representation may also be used to prove the existence of any facts asserted in that representation (Evidence Act 2008 s60).
  29. It has been suggested that, due to the potential unfairness that may follow from the application of this rule, it may be preferable to avoid admitting evidence of out-of-court representations solely to explain the assumptions on which an opinion is based (R v Lawson [2000] NSWCCA 214 per Sperling J).
  30. If such evidence is admitted, it may be desirable to use the discretion in s136 to limit the use that may be made of such evidence (Roach v Page (No 11) [2003] NSWSC 907. But see Harrington-Smith v Western Australia (No 2) (2003) 130 FCR 424; Alphapharm Pty Ltd v H Lundbeck A/S [2008] FCA 559; Bodney v Bennell (2008) 167 FCR 84).

    Use of Opinion Evidence

  31. The precise way in which the jury may use opinion evidence will vary depending on the nature of the evidence given and the purpose for which it was admitted (see "Admissibility of Opinion Evidence" above).
  32. This topic addresses the uses that may be made of opinion evidence that is admitted under Evidence Act 2008 s79 (expert opinion evidence), s78 (lay opinion evidence) and s77 (evidence admitted for a different purpose).

    Expert Opinion Evidence

  33. The jury may use expert opinion evidence:[9]
  34. Where an expert adopts an industry text or journal as an authoritative source, the jury may use any information contained in the text or journal as facts in the case if:

    Lay Opinion Evidence

  35. Lay opinion evidence[10] may help the jury to understand the matter or event about which the witness gave evidence (see, e.g., R v Leung (1999) 47 NSWLR 405).

    Opinion Evidence Admitted for a Different Purpose

  36. If evidence of an opinion is relevant and admissible for a purpose other than proving the existence of the fact which is the subject of the opinion, the exclusionary opinion rule (s76) does not apply (Evidence Act 2008 s77).[11]
  37. In such cases, the evidence may also be used to prove the existence of the fact which is the subject of the opinion (Allstate Life Insurance Co v ANZ Banking Group Ltd (No 5) (1996) 64 FCR 73; Hughes Aircraft Systems International v Airservices Australia (1997) 80 FCR 276).
  38. In some cases it will be undesirable for the evidence to be used in this way. In such circumstances it may be appropriate to use the discretion in s136 to limit the use that may be made of such evidence (see, e.g., Murex Diagnostics Australia Pty Ltd v Chiron Corporation (1995) 62 FCR 424; Perpetual Trustee Company Ltd v George; Estate of Conacher (No 2) 28/11/97 NSW SC; Roach v Page (No 11) [2003] NSWSC 907; James v Launceston City Council (2004) 13 Tas R 89; [2004] TASSC 69).

    Jury Directions

  39. The need for a direction on opinion evidence depends on whether a direction is sought or whether there are substantial and compelling reasons for giving a direction in the absence of a request (Jury Directions Act 2015 ss14 - 16). See Directions Under Jury Directions Act 2015 for information on when directions are required. The following sections describe the content of directions, if the judge gives directions on opinion evidence.
  40. The content of the jury directions will depend on the nature of the evidence given and the ways in which the jury may use that evidence (see "Use of Opinion Evidence" above).
  41. No guidance has yet been provided about the directions to be given in relation to lay opinion evidence. The remainder of this topic therefore only addresses directions to be given in relation to expert opinion evidence.

    Directions About Expert Opinion Evidence

  42. Expert opinion often involves unfamiliar and technical matters. Judicial directions should ensure that the jury can understand the evidence and apply it to the facts of the case (Velevski v R (2002) 187 ALR 233; R v Gemmill (2004) 8 VR 242).
  43. The judge’s directions on opinion evidence should explain the significance of the evidence in the context of the case, and give any special directions needed to enable the jury to assess and use the evidence that has been led (Taylor v R (1978) 22 ALR 599).
  44. The judge should ensure that the jury understands that, generally, only an expert is permitted to give evidence of his or her opinion. Other witnesses are generally limited to giving evidence of their own observations (Taylor v R (1978) 22 ALR 599; Farrell v R (1998) 194 CLR 286; Ramsay v Watson (1961) 108 CLR 642).
  45. Where an expert witness has given evidence about the "ultimate issue"[12], the judge should make it clear that it is the jury’s role to determine the issue (Australian Law Reform Commission, Evidence(Interim), Report 26 (1985) vol.1 para 743. See also Idoport Pty Ltd v National Australia Bank Ltd (2000) 50 NSWLR 640).
  46. It is for the jury to decide whether an opinion is credible and what weight it should be given (R v Anderson (2000) 1 VR 1; Velevski v R (2002) 187 ALR 233).
  47. The judge should tell the jury that they are entitled to reject the evidence if they are not satisfied that the science or the testing is sufficiently accurate, reliable or dependable. The jury is not bound by the opinion of experts, and must not be overawed by the scientific appearance of their opinions (R v Pantoja (1998) 88 A Crim R 554; R v Karger (2002) 83 SASR 135; United States v Baller (1975) 519 Fed 2d 463; R v Gilmore [1977] 2 NSWLR 935; R v Duke (1979) 22 SASR 46; R v Kotzmann [1998] 2 VR 123; R v Parker VicCA 10/8/1995).
  48. Where a witness has been cross-examined about the nature and quality of his or her expertise, the jury should be told that a witness’ competency and credit are relevant matters to take into account when determining the weight to be given to his or her evidence (see, e.g., Polycarpou v Australian Wire Industries Pty Ltd (1995) 36 NSWLR 49).
  49. The jury should be told that expert evidence is only as valuable as the facts supporting the evidence. That evidence only has probative value if the jury accepts the facts that form the basis of the evidence (R v Kotzmann [1998] 2 VR 123).
  50. The jury should be directed to examine the basis on which the expert formed his or her opinion, and to determine whether the facts constituting the basis have been proven (Nguyen v R (2007) 173 A Crim R 557).
  51. The judge should tell the jury to consider the following matters when they evaluate the evidence of experts:

    Unanimous Expert Evidence

  52. Where there is unanimous agreement amongst expert witnesses, the jury should be told that they are not bound by the experts’ opinion. However, they may only reject that evidence if:

    Conflicting Expert Evidence

  53. Where the evidence of expert witnesses conflicts, the jury should be told that they are entitled to prefer the evidence of one expert over another. The role of the jury is to select the evidence that they shall accept, and this includes expert evidence (Velevski v R (2002) 187 ALR 233; Chamberlain v R (No 2) (1983) 153 CLR 521; R v Gemmill (2004) 8 VR 242; R v GK (2001) 53 NSWLR 317; R v NCT (2009) 26 VR 247).
  54. It is therefore not appropriate for the judge to direct the jury that one expert is superior to another. This intrudes on the role of the jury (R v Gemmill (2004) 8 VR 242).
  55. In exceptional cases, the jury may be incapable of resolving a conflict between experts on matters of science. The judge should tell the jury that they must not accept disputed scientific evidence that is unfavourable to the accused unless there is a good reason to reject the defence evidence (Velevski v R (2002) 187 ALR 233).
  56. Where the jury’s decision about which expert to accept is likely to determine the accused’s guilt or innocence, the judge may tell the jury that they may only accept the evidence of the expert who is adverse to the accused if they are satisfied beyond reasonable doubt that his or her opinion is correct (R v Anderson (2000) 1 VR 1; R v Sodo (1975) 61 Cr App R 131).
  57. A direction that the jury must be satisfied beyond reasonable doubt that the prosecution expert is correct should not be given in all cases. Such a direction is only required when there are conflicting experts, and resolution of that conflict by the jury is likely to determine the case against the accused. In other cases the direction should not be given, as it will tend to isolate one piece of evidence and invite the jury to decide the case solely on the basis of that evidence (R v Middleton [2000] WASCA 213; R v Anderson (2000) 1 VR 1; R v Nicholas [1989] Tas R (NC) N24).

    Expert Evidence About Witness Credibility or Reliability

  58. Where expert evidence about a witness’ reliability is given, the judge should clearly direct the jury that they must not allow the expert to usurp their function of assessing the credibility of witnesses (Farrell v R (1998) 194 CLR 286).
  59. If the evidence of the expert is limited to suggesting that the witness may suffer a disorder that affects his or her reliability, the judge may comment on the limited probative value of that evidence (Farrell v R (1998) 194 CLR 286).

    Expert Evidence About Mental Impairment

  60. Expert evidence will usually be necessary to establish a defence under s20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997.
  61. In such cases, the judge should relate the expert evidence to the test for mental impairment (Taylor v R (1978) 22 ALR 599; Mizzi v R (1960) 105 CLR 659. See Mental Impairment).
  62. While the jury is not bound by the expert evidence, the judge must ensure that they do not ignore unchallenged expert evidence and substitute their common sense view of the evidence (R v Wiese [1969] VR 953; R v Gemmill (2004) 8 VR 242).
  63. The judge should warn the jury that, when assessing the expert evidence, they should not assume that the accused would reason in the same way as a person without a mental illness. Common assumptions about sane behaviour may be wrong, and the jury should carefully consider any expert evidence (R v Gemmill (2004) 8 VR 242; Taylor v R (1978) 22 ALR 599; R v Matusevich & Thompson [1976] VR 470; Mizzi v R (1960) 105 CLR 659; R v Weise [1969] VR 953).

    Expert Evidence About Child Sexual Abuse

  64. A person who has specialised knowledge about child development and child behaviour may give evidence relating to the development and behaviour of children generally, as well as about the development and behaviour of children who have been victims of sexual offences (or offences similar to sexual offences) (Evidence Act 2008 s79(2)).
  65. This evidence may be relevant to a range of matters in a trial, including testimonial capacity, the credibility of a child witness, the beliefs and perceptions held by a child, and the reasonableness of those beliefs and perceptions (Explanatory Memoranda to the Evidence Act 2008).
  66. This evidence may also assist the jury to assess other evidence in the case, or to address misconceived notions about children and their behaviour (Explanatory Memoranda to the Evidence Act 2008).
  67. Such evidence should usually only relate to the general behaviour and development of children who are victims of sexual offences. The evidence is designed to educate the jury and correct erroneous beliefs by showing that “counter-intuitive” behaviour does not detract from the complainant’s credibility. However, it will rarely be appropriate for a witness to give opinion evidence on whether the complainant’s behaviour makes it more likely that the complainant has been the victim of a sexual offence (MA v R (2013) 40 VR 564).
  68. However, the admission of such evidence might invite the jury to improperly reason as follows:
  69. It may therefore be appropriate in cases where such evidence is admitted to direct the jury not to reason in this way (Australian Law Reform Commission Report 102, Uniform Evidence Law, para 9.157).

     

    Notes

[1] See "Admissibility of Opinion Evidence" below for more information about the opinion rule.

[2] For example, where a witness with experience of how a particular type of vehicle behaves in certain conditions gives evidence of that experience, it will not be opinion evidence (Clark v Ryan (1960) 103 CLR 486).

[3] Whether or not this type of evidence will be opinion evidence will depend on the precise nature of the evidence given. It will not be opinion evidence where it is seen to be evidence of fact about professional practices or professional standards.

[4] See Identification Evidence for a discussion of the many risks inherent in the process of identification.

[5] Section 76 states: "Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed."

[6] This includes specialised knowledge of child development and child behaviour, including specialised knowledge of the impact of sexual abuse on children and their development and behaviour during and following the abuse (Evidence Act 2008 s79(2)).

[7] For example, where the evidence is admitted to establish the consistency of the witness’ conduct, rather than the truth of his or her assertions.

[8] As this evidence is not being admitted to prove the existence of the facts asserted by the representation, it is not captured by the hearsay rule: Evidence Act 2008 s59(1).

[9] Evidence that is wholly or substantially based on specialised knowledge that the witness has obtained from training, study or experience (Evidence Act 2008 s79).

[10] Evidence that is based on what the witness saw, heard or otherwise perceived about a matter or event, which is necessary to obtain an adequate account or understanding of the witness’ perception of the matter or event (Evidence Act 2008 s78).

[11] For example, opinion evidence may be admitted to establish the consistency of a witness’ conduct, rather than the truth of his or her assertions.

[12] As the ultimate issue rule has been abolished (Evidence Act 2008 s80), such evidence may now be admissible. See "Abolition of Common Knowledge and Ultimate Issue Rules" above.

Last updated: 29 June 2015

In This Section

4.14.1.1 - Charge: Uncontested Expert Evidence

4.14.1.2 - Charge: Contested Expert Evidence

4.14.1.3 - Charge: Lay Opinion Evidence

See Also

4.14 - Opinion Evidence

4.14.2 - DNA Evidence

4.14.3 - Fingerprint Evidence

4.14.4 - Handwriting Evidence