**6. The Chartered Society of Forensic Sciences and the College of Podiatry Code of Practice**

Importantly, in 2020 the Chartered Society of Forensic Science and the College of Podiatry [24] published a 'Code of Practice for Forensic Gait Analysis'. It stipulated that the method used for the preliminary assessment in forensic gait analysis should include assessment of fact such as (but not limited to):

1.distortions of the image inherent in the footage;

2.the resolution (sharpness), lighting and frame rate of the footage;


The Code also stated that features of gait should only be compared if the locomotor activity being undertaken in the questioned footage is the same as that being undertaken in the reference footage – walking footage should be compared with walking footage. The comparison should consider features of gait that:


In addition, the Code of Practice ([24] at [18.63]–[18.68]) stated that:

	- It is available for use by both the prosecution and defence.
	- States the size of the population used.
	- States the appropriateness to the case of the population used.

**17**

*Guarding the Gait: Evaluating Forensic Gait Analysis Evidence*

final report, and the database identified.

numerical data or statistical calculation.

the expert to proffer that opinion.

**7. Status of gait analysis evidence**

occupies screen height [23].

estimated and evaluated.'

of the statistical significance of any finding of similarity.

• If a database has been used to assist in the determination of the strength of evidence provided by the forensic gait analysis this shall be made clear in the

• In the absence of the use of a database the final report shall contain a statement to the effect that the determination of the strength of evidence provided by the forensic gait analysis is an opinion based conclusion, and is not predicated on

• Where the expert provides an opinion based solely on their experience it is important that the statement make clear, in detail, the experience which allows

• The strength of evidence provided by the forensic gait analysis should be

questioned footage is not the subject in the reference footage'.

expressed using a published scale of verbal expressions of strength of evidence in support of one of two opposing propositions. Generally, the prosecution's proposition would take the form of 'the figure in the questioned footage is the subject in the reference footage'. The alternative proposition is determined by the defence but in the majority of cases takes the form of 'the figure in the

The Code of Practice for Forensic Gait Analysis is not binding outside the United Kingdom but it constitutes a sound articulation of responsible practice in a period when the area of forensic gait analysis is moving toward a sounder scientific basis.

For the present the area of forensic gait analysis poses significant risks in terms of its reliability and the danger that it will be invested with greater respect than its current scientific status deserves in light of the subjectivity inherent in most of its evaluations and the absence of rigorous databases which might enable a statement

In addition, it is significant that the quality of CCTV footage varies considerably. This can affect the ability of forensic gait analysts to undertake their evaluations informedly. For instance, frame rates on CCTV footage vary markedly, from one frame every 4 seconds to 25 frames per second. The work of Birch et al. [22] has shown that this variation has a major impact upon the ability of even experienced professionals to identify characteristics of gait with accuracy. While they have commented that, 'Every effort should therefore be made to ensure that CCTV footage likely to be used in criminal proceedings is captured at as high a frame rate as possible' ([22] at 169), this has not resolved the point at which such analysis ceases or commences to be reliable. A further issue potentially affecting such capacity goes to the extent to which the figure targeted

Lynnerup and Larsen [25] have contended that: 'exact photogrammetric measurements may be made from CCTV material of perpetrators and at the scene of the crime, but that care must be taken to ensure that error ranges, especially connected to measuring the human (clothed and masked) body in motion, are critically

The decisions in the Canadian *Aitken* case and the Court of Appeal decisions in *Otway* and *Hashi* have extended significant and problematic latitude to forensic gait

*DOI: http://dx.doi.org/10.5772/intechopen.99491*


*Guarding the Gait: Evaluating Forensic Gait Analysis Evidence DOI: http://dx.doi.org/10.5772/intechopen.99491*

*Forensic Analysis - Scientific and Medical Techniques and Evidence under the Microscope*

3.the locomotor activity being undertaken by the figure/subject;

4.the number of consecutive mid gait steps seen in the footage;

6.the direction in which the figure/subject is moving relative to the camera;

8.the possible impact on gait associated with the figure/subject's footwear or lack

9.the possible impact on gait associated with the figure/subject's environment.

The Code also stated that features of gait should only be compared if the locomotor activity being undertaken in the questioned footage is the same as that being undertaken in the reference footage – walking footage should be compared with

a.are exhibited by both the figure in the questioned footage and the subject in the

b.would preclude the figure in the questioned footage from being the subject in

c. are exhibited by the figure in the questioned footage, but not the subject in the reference footage, but do not preclude the figure in the questioned footage from

d.are exhibited by the subject in the reference footage, but not the figure in the questioned footage, but do not preclude the subject in the reference footage

• A database can be used to assist in the determination of the strength of evidence by the forensic gait analysis; however, its admissibility may be ques-

In addition, the Code of Practice ([24] at [18.63]–[18.68]) stated that:

tioned if the database does not meet all of the following criteria:

○ It is available for use by both the prosecution and defence.

○ States the appropriateness to the case of the population used.

• The likelihood of such a database being deemed admissible may be

walking footage. The comparison should consider features of gait that:

being the subject in the reference footage; and

from being the figure in the questioned footage.

○ States the size of the population used.

○ it is also in the public domain; and/or

○ has been peer reviewed and published.

7.the relative size of the image of the figure/subject in the field of view;

5.the position of the camera relative to the figure/subject;

of footwear; and

reference footage;

the reference footage;

**16**

boosted if:


The Code of Practice for Forensic Gait Analysis is not binding outside the United Kingdom but it constitutes a sound articulation of responsible practice in a period when the area of forensic gait analysis is moving toward a sounder scientific basis.

### **7. Status of gait analysis evidence**

For the present the area of forensic gait analysis poses significant risks in terms of its reliability and the danger that it will be invested with greater respect than its current scientific status deserves in light of the subjectivity inherent in most of its evaluations and the absence of rigorous databases which might enable a statement of the statistical significance of any finding of similarity.

In addition, it is significant that the quality of CCTV footage varies considerably. This can affect the ability of forensic gait analysts to undertake their evaluations informedly. For instance, frame rates on CCTV footage vary markedly, from one frame every 4 seconds to 25 frames per second. The work of Birch et al. [22] has shown that this variation has a major impact upon the ability of even experienced professionals to identify characteristics of gait with accuracy. While they have commented that, 'Every effort should therefore be made to ensure that CCTV footage likely to be used in criminal proceedings is captured at as high a frame rate as possible' ([22] at 169), this has not resolved the point at which such analysis ceases or commences to be reliable. A further issue potentially affecting such capacity goes to the extent to which the figure targeted occupies screen height [23].

Lynnerup and Larsen [25] have contended that: 'exact photogrammetric measurements may be made from CCTV material of perpetrators and at the scene of the crime, but that care must be taken to ensure that error ranges, especially connected to measuring the human (clothed and masked) body in motion, are critically estimated and evaluated.'

The decisions in the Canadian *Aitken* case and the Court of Appeal decisions in *Otway* and *Hashi* have extended significant and problematic latitude to forensic gait analysis evidence, regarding the area as one of expertise and sufficient to enable jurors to utilise it as part of their task of evaluating for themselves similarities in different forms of footage of a person of interest and the accused. The ruling by Justice Beale in the Australian case of *Crupi* stands as a contrast to these authorities with his Honour being disinclined to permit opinion of 'remarkable similarity' on multiple bases including relevance, whether the expert was an expert in the relevant area, its constituting tendency evidence, and its being more prejudicial than probative.

A 2017 report by the Royal Society and the Royal Society of Edinburgh, in conjunction with the Judicial College, the Judicial Institute and the Judicial Studies Board for Northern Ireland ([5] at p6) supported the more cautious approach embraced by Justice Beale, warning that:

*Its underpinning science is sparse and largely translated from the more developed fields of clinical gait analysis and biomechanics, with more recent insights from biometrics. Care is required, however, in assuming that techniques developed in one field can be applied in another with quite different objectives. The scientific evidence supporting forensic gait analysis, as currently practised, is thus extremely limited.*

*When forensic gait analysis is used as an aid to positive identification of a suspect, the following matters should be borne in mind:*

*There is no evidence to support the assertion that gait is unique within current or foreseeable limitations of measurements used in forensic gait analysis.*

*There is no credible database currently that permits assessment of the frequency of either normal or abnormal gait characteristics.*

*There are no published and verified error rates associated with the current methodology.*

*There are no published black-box studies of analyst reliability and repeatability. There is no standardised methodology for analysis, comparison and reporting of gait characteristics.*

Cunliffe and Edmond [10, 26] have been particularly concerned about the current state of the art of forensic gait analysis and of forensic podiatry in general. They have argued powerfully that for the present the evidence is inherently unreliable and urged application of stringent tests to exclude it in most cases from jurors' consideration. They have contended ([24] at 279) that it

*might be useful as a technology capable of assisting with identification or exclusion. This, however, assumes that technical problems associated with validation, image quality, duration of view, frame rates, different types of movement (e.g., walking, running, dancing, carrying objects, moving with injuries, moving while intoxicated, trying to disguise gait, or walking toward somebody with the intention of shooting), frequency and interrelatedness of features, and cognitive bias can be overcome. As things stand, forensic gait analysis can merely suggest that a person could be included within a set of similar persons where the apparent or alleged similarities are of unknown frequency, so the size of the set is unknown.*

Van Matrigt et al. [11] have correctly identified that: 'To improve clarity on admission of gait as evidence and assessing its evidential value, method validity and reliability and expert proficiency should be reported', a position supported by Cunliffe and Edmond. Van Matrigt et al. [11] have also argued that forensic gait analysts should:

**19**

nal trials.

**Conflict of interest**

*Guarding the Gait: Evaluating Forensic Gait Analysis Evidence*

data collection procedures and algorithm development.'

statisticians, to evaluate the accurate interpretation of results.

The author declares no conflict of interest.

*join forces to create an international standard forensic gait analysis method with known validity, reliability and proficiency tests. We propose to focus on designing and publishing on large (inter)national gait databases and methods for likelihood calculation taking into account interdependent features. We also hope for (inter) national guidelines for the admission of forensic gait analysis in court. This is especially important since forensic gait analyst is not a protected professional title.*

Macoveciuc et al. [27] have emphasised the increasing role of digitisation of matching procedures within forensic podiatry and usefully argued that: 'A collaboration should be established between the gait analyst and the biometric specialist where the analyst inputs their own expertise to assess the match concluded by the biometric system and also advises (from a clinical and forensic perspective) on the

While forensic podiatry in particular and forensic gait analysis have enthusiasts [28] and have the potential to provide assistance to investigators and potentially to the courts, the harder question is as to the appropriateness of forensic gait analysis as expert opinion evidence until further development in the discipline takes place. Undeniably, there are risks in the subjectivity of the comparison process which has the potential to be distorted by cognitive and contextual biases. There is an urgent need for the development of relevant databases so that subjective identification of what is unusual, out-of-the-ordinary or remarkable can be contextualised objectively and, in particular, numerically [29]. Work in this regard has commenced but as yet has some distance to travel. It remains to be seen whether the area remains the principal preserve of forensic podiatrists and whether medical practitioners have a constructive contribution that they can make to its development. In all probability, though, as in a number of areas of forensic science, contributions can be made by practitioners from diverse disciplines, including biometric experts and forensic

For the present what needs to be observed is that there are real questions about the probative value of most forensic gait analysis evidence, that expert evidence in respect of comparison of images needs to be undertaken with temperance and circumspection and that the prejudicial value of such evidence needs to be subjected to rigorous evaluation, including by reference to the Code of Practice for Forensic Gait Analysis published in 2020 by the Chartered Society of Forensic Science and the College of Podiatry [24] to determine whether it should be admitted in crimi-

*DOI: http://dx.doi.org/10.5772/intechopen.99491*

*Forensic Analysis - Scientific and Medical Techniques and Evidence under the Microscope*

analysis evidence, regarding the area as one of expertise and sufficient to enable jurors to utilise it as part of their task of evaluating for themselves similarities in different forms of footage of a person of interest and the accused. The ruling by Justice Beale in the Australian case of *Crupi* stands as a contrast to these authorities with his Honour being disinclined to permit opinion of 'remarkable similarity' on multiple bases including relevance, whether the expert was an expert in the relevant area, its constituting tendency evidence, and its being more prejudicial than

A 2017 report by the Royal Society and the Royal Society of Edinburgh, in conjunction with the Judicial College, the Judicial Institute and the Judicial Studies Board for Northern Ireland ([5] at p6) supported the more cautious approach

*Its underpinning science is sparse and largely translated from the more developed fields of clinical gait analysis and biomechanics, with more recent insights from biometrics. Care is required, however, in assuming that techniques developed in one field can be applied in another with quite different objectives. The scientific evidence supporting forensic gait analysis, as currently practised, is thus extremely* 

*When forensic gait analysis is used as an aid to positive identification of a suspect,* 

*There is no evidence to support the assertion that gait is unique within current or* 

*There is no credible database currently that permits assessment of the frequency of* 

*There are no published black-box studies of analyst reliability and repeatability. There is no standardised methodology for analysis, comparison and reporting of* 

Cunliffe and Edmond [10, 26] have been particularly concerned about the current state of the art of forensic gait analysis and of forensic podiatry in general. They have argued powerfully that for the present the evidence is inherently unreliable and urged application of stringent tests to exclude it in most cases from jurors'

*might be useful as a technology capable of assisting with identification or exclusion. This, however, assumes that technical problems associated with validation, image quality, duration of view, frame rates, different types of movement (e.g., walking, running, dancing, carrying objects, moving with injuries, moving while intoxicated, trying to disguise gait, or walking toward somebody with the intention of shooting), frequency and interrelatedness of features, and cognitive bias can be overcome. As things stand, forensic gait analysis can merely suggest that a person could be included within a set of similar persons where the apparent or alleged similarities are of unknown frequency, so* 

Van Matrigt et al. [11] have correctly identified that: 'To improve clarity on admission of gait as evidence and assessing its evidential value, method validity and reliability and expert proficiency should be reported', a position supported by Cunliffe and Edmond. Van Matrigt et al. [11] have also argued that forensic gait

*foreseeable limitations of measurements used in forensic gait analysis.*

*There are no published and verified error rates associated with the current* 

**18**

analysts should:

probative.

*limited.*

*methodology.*

*gait characteristics.*

*the size of the set is unknown.*

embraced by Justice Beale, warning that:

*the following matters should be borne in mind:*

*either normal or abnormal gait characteristics.*

consideration. They have contended ([24] at 279) that it

*join forces to create an international standard forensic gait analysis method with known validity, reliability and proficiency tests. We propose to focus on designing and publishing on large (inter)national gait databases and methods for likelihood calculation taking into account interdependent features. We also hope for (inter) national guidelines for the admission of forensic gait analysis in court. This is especially important since forensic gait analyst is not a protected professional title.*

Macoveciuc et al. [27] have emphasised the increasing role of digitisation of matching procedures within forensic podiatry and usefully argued that: 'A collaboration should be established between the gait analyst and the biometric specialist where the analyst inputs their own expertise to assess the match concluded by the biometric system and also advises (from a clinical and forensic perspective) on the data collection procedures and algorithm development.'

While forensic podiatry in particular and forensic gait analysis have enthusiasts [28] and have the potential to provide assistance to investigators and potentially to the courts, the harder question is as to the appropriateness of forensic gait analysis as expert opinion evidence until further development in the discipline takes place. Undeniably, there are risks in the subjectivity of the comparison process which has the potential to be distorted by cognitive and contextual biases. There is an urgent need for the development of relevant databases so that subjective identification of what is unusual, out-of-the-ordinary or remarkable can be contextualised objectively and, in particular, numerically [29]. Work in this regard has commenced but as yet has some distance to travel. It remains to be seen whether the area remains the principal preserve of forensic podiatrists and whether medical practitioners have a constructive contribution that they can make to its development. In all probability, though, as in a number of areas of forensic science, contributions can be made by practitioners from diverse disciplines, including biometric experts and forensic statisticians, to evaluate the accurate interpretation of results.

For the present what needs to be observed is that there are real questions about the probative value of most forensic gait analysis evidence, that expert evidence in respect of comparison of images needs to be undertaken with temperance and circumspection and that the prejudicial value of such evidence needs to be subjected to rigorous evaluation, including by reference to the Code of Practice for Forensic Gait Analysis published in 2020 by the Chartered Society of Forensic Science and the College of Podiatry [24] to determine whether it should be admitted in criminal trials.
