STRmix expert

Authoritative forensic expert testimony in the disciplines of DNA, bloodstain pattern analysis (BPA), STRmix and biological fluids.

A preliminary review can be a useful step in assessing the broad strengths and weaknesses of a particular matter. This process can also identify documentary and any further evidence requirements.

I rigorous interrogation of the evidence will uncover underlying issues and determine the most appropriate pathway towards reviewing and reporting the matter.

Certain complex matters benefit from a draft report, which can open areas for discussion with Counsel, and potentially allow for defence to approach the prosecution.

Following a thorough evaluation of the evidence, a DNA expert report will be issued in accordance with the expert witness code of conduct. The report will be suitable for submission in evidence.

Preparations with Counsel are often conducted, such that the probative value of the evidence is weighed effectively and persuasively at Voir Dire, should such a hearing be required.

Extensive preparations are generally conducted in anticipation of substantive hearing. Which may include cross examination and evidence in chief scenarios specific to the matter .

STRmix EXPERT

STRmix expert review correctly follows a stringent and thorough path including consideration of the various subjective analyst determinations relied upon.

STRmix is utilised by all government forensic laboratories in Australia and New Zealand, as well as in over 100 forensic laboratories worldwide including USA, UK, Ireland, Canada, Finland, Switzerland and Denmark.

 

STRmix applies a fully continuous probabilistic genotyping method, conducting DNA likelihood ratio calculations on single source and mixed DNA profiles.

The software utilises statistical methodology and data regarding a range of DNA profile behaviours, conducting hundreds of thousands of random simulations.

It is critical to understand that the resultant likelihood ratio is not at all a likelihood that the contributor came into contact with the item from which the profile was obtained.

STRmix has been extensively validated for use, however there are known limitations to the software which must be considered when evaluating the data generated.

STRmix expert review correctly follows a stringent and thorough path including consideration of the various subjective analyst determinations relied upon.

STRmix is utilised by all government forensic laboratories in Australia and New Zealand, as well as in over 100 forensic laboratories worldwide including USA, UK, Ireland, Canada, Finland, Switzerland and Denmark.

 

STRmix applies a fully continuous probabilistic genotyping method, conducting DNA likelihood ratio calculations on single source and mixed DNA profiles.

The software utilises statistical methodology and data regarding a range of DNA profile behaviours, conducting hundreds of thousands of random simulations.

It is critical to understand that the resultant likelihood ratio is not at all a likelihood that the contributor came into contact with the item from which the profile was obtained.

STRmix has been extensively validated for use, however there are known limitations to the software which must be considered when evaluating the data generated.

STRmix False Inclusion

A false inclusion provides inclusionary support of an individual, when that individual has not contributed to the DNA profile.
A STRmix false inclusion can be considered on factors including inputs, outputs, diagnostics and propositions.
Significant contributing factors to STRmix false inclusions are the subjective decisions that the STRmix analyst makes when conducting the interpretation.
An empirical study using STRmix simulated 300,000 mixed DNA profiles with false inclusions occurring on 2900 occasions.
Known contributing factors of false inclusion are DNA profile complexity, number of contributors, and allele sharing.

DNA Likelihood ratio

The DNA likelihood ratio (LR) is an internationally recognised numerical method to evaluate mixed DNA profiles.
Various methods can be used to formulate LR, including STRmix.
The likelihood ratio number could range up to 100 billion, and greater in certain jurisdiction.
The LR is a consideration of the likelihood of obtaining the observed DNA profile given two hypotheses, which should be the prosecution and defense propositions,
Valid DNA likelihood ratio calculation can turn on analyst inputs, including choice of propositions, allele frequency rate, theta correction, ethnicity considerations and relatives of the individual.

DNA Contamination

DNA contamination can affect evidence from crime scene, laboratory and throughout the entire chain of custody.
Such contamination can impact upon the STRmix assessment.
Increased testing sensitivity means the majority of DNA profiles contain levels of background DNA. Background DNA will be analysed during the STRmix assessment and is commonly defined as an unknown contributor.
Background DNA of unknown contributors can represent contamination.
A rigorous review of the STRmix outputs, along with the laboratory contamination report can assist in the identification of any contamination that may have contributed to the STRmix assessment.

STRmix Error Rate

STRmix error rate, known STRmix miscoding, and the variable nature of STRmix outputs, have been the subject of divisive scientific and legal debate through the evolution of STRmix and other PG (probabilistic genotyping) software development.
Multiple STRmix likelihood ratios will commonly be generated, and the analyst must exercise skill in the required subjective selection.
STRmix error reduction is reliant upon the appropriate formulation of propositions, contributor assigned and parameter settings, amongst other factors.
There are currently no published guidelines on the acceptable STRmix error rate.

DNA Ethnicity database

Valid DNA likelihood ratio calculation relies upon the appropriate ethnicity database selection.
In Australia, commonly selected datasets are Australian Aboriginal, Caucasian, Asian and Sudanese, though there are many others that can be utilised.
The numerical value (ie 100 billion) assigned to a DNA profile, requires an estimation as to how “common” or “rare” the profile is.
If the same DNA profile is assessed by STRmix using three different ethnicity databases’, it will produce three different likelihood ratios.
Certain laboratories have a tendency of applying a specific ethnicity, and others approach the matter on a case-by-case basis.

Number of Contributors

A primary limitation of STRmix is that the analyst estimates the number of contributors.
Incorrect estimation of the number of contributors can directly cause an invalid likelihood ratio.
Contributor selection is critically reliant upon the DNA analyst’s skill, experience, and ability to evaluate factors of allele sharing, allele dropout, and stutter.
To minimise risk of bias, best practice state that the analyst conducts contributor selection without consideration to the person of interest samples.
Interrogation of the STRmix primary and secondary diagnostic outputs can assist to determine if the likelihood ratio is valid.
False inclusion

STRmix False Inclusion

A false inclusion provides inclusionary support of an individual, when that individual has not contributed to the DNA profile.
A STRmix false inclusion can be considered on factors including inputs, outputs, diagnostics and propositions.
Significant contributing factors to STRmix false inclusions are the subjective decisions that the STRmix analyst makes when conducting the interpretation.
An empirical study using STRmix simulated 300,000 mixed DNA profiles with false inclusions occurring on 2900 occasions.
Known contributing factors of false inclusion are DNA profile complexity, number of contributors, and allele sharing.
Likelihood ratio

DNA Likelihood ratio

The DNA likelihood ratio (LR) is an internationally recognised numerical method to evaluate mixed DNA profiles.
Various methods can be used to formulate LR, including STRmix.
The likelihood ratio number could range up to 100 billion, and greater in certain jurisdiction.
The LR is a consideration of the likelihood of obtaining the observed DNA profile given two hypotheses, which should be the prosecution and defense propositions,
Valid DNA likelihood ratio calculation can turn on analyst inputs, including choice of propositions, allele frequency rate, theta correction, ethnicity considerations and relatives of the individual.
Contamination

DNA Contamination

DNA contamination can affect evidence from crime scene, laboratory and throughout the entire chain of custody.
Such contamination can impact upon the STRmix assessment.
Increased testing sensitivity means the majority of DNA profiles contain levels of background DNA. Background DNA will be analysed during the STRmix assessment and is commonly defined as an unknown contributor.
Background DNA of unknown contributors can represent contamination.
A rigorous review of the STRmix outputs, along with the laboratory contamination report can assist in the identification of any contamination that may have contributed to the STRmix assessment.
Error rate

STRmix Error Rate

STRmix error rate, known STRmix miscoding, and the variable nature of STRmix outputs, have been the subject of divisive scientific and legal debate through the evolution of STRmix and other PG (probabilistic genotyping) software development.
Multiple STRmix likelihood ratios will commonly be generated, and the analyst must exercise skill in the required subjective selection.
STRmix error reduction is reliant upon the appropriate formulation of propositions, contributor assigned and parameter settings, amongst other factors.
There are currently no published guidelines on the acceptable STRmix error rate.
Ethnicity database

DNA Ethnicity database

Valid DNA likelihood ratio calculation relies upon the appropriate ethnicity database selection.
In Australia, commonly selected datasets are Australian Aboriginal, Caucasian, Asian and Sudanese, though there are many others that can be utilised.
The numerical value (ie 100 billion) assigned to a DNA profile, requires an estimation as to how “common” or “rare” the profile is.
If the same DNA profile is assessed by STRmix using three different ethnicity databases’, it will produce three different likelihood ratios.
Certain laboratories have a tendency of applying a specific ethnicity, and others approach the matter on a case-by-case basis.
Number of contributors

Number of Contributors

A primary limitation of STRmix is that the analyst estimates the number of contributors.
Incorrect estimation of the number of contributors can directly cause an invalid likelihood ratio.
Contributor selection is critically reliant upon the DNA analyst’s skill, experience, and ability to evaluate factors of allele sharing, allele dropout, and stutter.
To minimise risk of bias, best practice state that the analyst conducts contributor selection without consideration to the person of interest samples.
Interrogation of the STRmix primary and secondary diagnostic outputs can assist to determine if the likelihood ratio is valid.

STRmix QUESTIONS

What is STRmix

Developed by the Institute of Environmental Science and Research (ESR), STRmix™ uses biological modeling and mathematical processes to resolve complex DNA mixtures. By utilising likelihood ratios (LRs) to quantify evidence for a hypothesis in a continuous approach, STRmix™ delivers faster, more accurate responses than binary methods previously in use.  STRmix was launched in 2012, and has produced legally admissible DNA evidence in more than 385,000 criminal cases worldwide. All Australian and New Zealand government laboratories use STRmix™ to analyse DNA evidence in criminal matters.

STRmix Likelihood Ratio

STRmix evaluates the DNA evidence generating a likelihood ratio (LR).  In simple terms, any LR greater than 1 provides support for the inclusion of the individual in question.  STRmix can also produce LRs supporting the exclusion of that individual from the DNA profile.

Who uses STRmix?

All Australian and New Zealand government laboratories use STRmix to analyse DNA evidence in criminal matters. STRmix™ is also being utilised in over 80 US laboratories in various regions, including Utah crime scene evidence and Alabama complex crime scene as well as in the United Kingdom and European facilities.

Access to STRmix

STRmix can be accessed by scientists engaged by the Defence.  Where neceesary we can conduct independent STRmix analyses, and further assess the results presented by the Prosecution.

STRmix false inclusions

When comparing a known non-donor DNA profile to the mixed DNA profile the STRmix software can sometimes produce a likelihood ratio supporting their contribution to the mixed DNA profile. In lay terms, STRmix™ can give a likelihood ratio that includes the accused as a contributor when in fact, they are not a contributor at all. This scenario is known as false inclusion.

 

False inclusion rates

STRmix false inclusion rates have been published in the STRmix developmental validation study, as well as other large scale studies.

 

Low likelihood ratios

The lower the likelihood ratio, the greater the risk of a false inclusion.

Proving a false inclusion

Determining whether a result is a false inclusion requires a skilled analyst.  Many factors must be considered, including the nature of the DNA profile, the number of contributors and the STRmix diagnostic results.

STRMix - Terms

Deconvolution

The process of separating a DNA profile into genotype sets and their associated probability weightings

Unified likelihood ratio

LR that takes into account that the unknown contributors within H1 (Hp) and H2 (Hd) are made of up of both relatives and unrelated people.

Stratified likelihood ratio

When multiple populations are selected to calculate an LR, STRmixTM will calculate LRs for each population individually and then provide a single LR that samples across all populations.

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