STRmix expert


STRmix™ Expert Witness

Our STRmix™ expert witness opinion has assisted with evidence involving 100’s of STRmix™ matters. Our depth of STRmix™ experience covers matters at first instance through to appeal, in various jurisdictions throughout Australia and internationally, before juries and judge only trials. Our opinions are robustly and thoroughly formed as well as being peer reviewed, using our extensive knowledge of the theory and the relevant supporting scientific data.

STRmix™ is an Australian and New Zealand developed software which facilitates the forensic analysis, interpretation and investigation of DNA profiles. Launched in 2012, STRmix™ analysis is an alternative to the binary interpretation method and has produced legally admissible DNA evidence in more than 375,000 criminal cases worldwide.


All Australian government laboratories use STRmix™ to analyse criminal matter involving DNA, as do New Zealand labs. STRmix™ is also being utilised by 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


Key issues identified

STRmix™ seeks to provide improved interpretation of DNA evidence which was previously seen as too complex and degraded to interpret. When using STRmix™, analysts are able to combine DNA profiles in the one interpretation so as to compare profiles against the subject person in order to produce a likelihood ratio calculation for the DNA mixture. The STRmix™ procedure, requires the forensic scientist to determine the parameters for each piece of input data. This determination process requires significant scientific skill and experience.

Independent Forensic Services is an Australian service provider equipped to review STRmix™ output data. This STRmix™ review can necessitate various processes including accessing the STRmix™ software platform to conduct an independent analysis of the subject profile in part or full, as required. Through rigorous interrogation of the STRmix ™ diagnostics and output data we are able to determine if the evidence should be appropriately challenged before the courts.


100 billion times more likely – does not prove guilt!

STRmix™ evidence seeks to produce statistically weighted evidence in the form of a numerical ratio. This ratio is intended to assist with understanding the likelihoods of observing the DNA profile obtained.


The STRmix™ likelihood ratio can be expressed in terms such as ‘it is greater than 100 billion times more likely to obtain this mixed profile….’. The inherent downside to this specific ratio, is that the courts and jury regularly misunderstand the intended meaning of the likelihoods. The lay person may assume that the likelihood ratio means ‘It is 100 billion times more likely to be the accused than anyone else’. This is definitely not the meaning.


Furthermore, just because a larger likelihood ratio has been produced, it may not mean there is significant evidential value, there may be other circumstances, such as trace DNA transfer and persistence and the attribution to biological fluids that may mean the evidence has little evidential weight.


Jury comprehension and understanding of DNA evidence has been a significant concern over the preceding decades within the justice system, partly as a consequence of the increased reliance on DNA. Studies have been conducted in an effort to better understand the possibility for improving jury comprehension


STRmix™ false inclusion

The majority of DNA profiles obtained are mixed DNA profiles, meaning they contain DNA from more than one individual.  The more contributors to a mixed DNA profile, inherently the more complex the DNA profile and associated interpretation is.  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.


STRmix false inclusion rates have been published in the STRmix developmental validation study.

False inclusion is of quite serious concern, as it could clearly lead to increased risk of false conviction.

Cases have been documented in which contaminated samples or misinterpretations of DNA tests have led to wrongful convictions (Hagan 2009; Thompson et al. 2003). As extensively experienced DNA experts, we are skilled to forensically dissect the evidence and identify increased risk of false inclusion within the DNA report. Our expert witness opinion in regards to false inclusion has been accepted by the prosecution and has been instrumental in bringing about the withdrawal of the DNA evidence.




Challenging STRmix™ evidence


Our principal scientist Jae Gerhard, has many years’ experience assessing DNA profile evidence. From the previous binary method, Independent Forensic Services has closely worked with the adaptation to STRmix™.


The inception of STRmix™ has seen a change in the way DNA profiles are assessed by government labs in Australia. This change has represented an overall improvement in level of detail produced, the breadth of suitable subject profiles, and has allowed this work to be completed more efficiently. STRmix™ has been used in combination with improved DNA profiling technologies to help clear historical cold cases. We find that STRmix™ is perceived broadly as providing a scientific conclusion that is entirely objective. This perception is not scientifically correct, as the matter is in fact more subjective.


We are highly regarded in our scientific field and are well equipped to determine inadequacies or inaccuracies within STRmix™ DNA evidence. Furthermore, our expert witness experience allows us to assist Solicitors and Barristers to understand the full depth of the matter, and assist with questions of utility and potential outcomes for a specific subject matter.


Q – Can STRmix™ evidence be challenged?

A – Yes it can. We always recommend a thorough review of STRmix™ evidence. This review process has led us to challenge STRmix™ DNA evidence before the courts.


Q – Are STRmix™ results worth challenging?

A – We always recommend a thorough review of STRmix™ results.

STRmix™ has limitations, which must be considered.  STRmix™ requires the technician to determine the number of contributors to the DNA profile, which is a subjective determination.  It also requires the analyst to interpret the output data. This means that factors such as skill, experience and confirmation bias can affect the results.


Q – What is a STRmix™ false inclusion?

A – It has been demonstrated through research that STRmix™ can produce inclusionary results for individuals that have not contributed to a DNA profile – this is termed a false inclusion. Our review of the STRmix™ data can assist in identifying if this has occurred.


Q – Is STRmix™ used in all Australian criminal matters?

A – All Australian government forensic laboratories utilise STRmix™ in criminal proceedings involving DNA evidence.


Q – How do Australian Courts view STRmix™ results?

A – STRmix™ has been in use for over 10 years and is widely used across Australia.  STRmix™ has been accepted into evidence over 380,000 times. Despite frequent use, STRmix™ evidence is regularly misunderstood by the legal profession and Courts. Our experience is that the Courts significantly benefit from skilled expert assistance to understand this evidence on a matter specific basis.


Q – Has STRmix™ has been used in my clients DNA report?

A – Yes, is the most likely answer. Australian government forensic laboratories utilise STRmix™ in most criminal proceedings involving DNA evidence.

If the DNA evidence is accompanied by a numerical statistical weighting, it is likely STRmix™ has been used to evaluate the DNA evidence. Terms such as ‘H1’, ‘H2’, or ‘the DNA profile is X times more likely to be obtained if…’ will likely indicate the use of STRmix™. We can determine with certainty if STRmix™ has been used in the specific matter.


Q – Does 100 billion times more likely, mean that the DNA profile is observed 1 in 100 billion people?

A – It absolutely does not. Australian government laboratory forensic reports commonly refer to the DNA profile being 100 billion times more likely……. This writing is very commonly misunderstood by lawyers, Courts and juries alike. We firmly believe that the Courts significantly benefit from our expert assistance to understand the true scientific and statistical meaning of the likelihood ratios.


Q – Can STRmix™ results incorporate bias?

A – Some STRmix™ input parameters are selected by the technician and are therefore subjective. Subjective scientific processes are inherently open to the different forms of bias.


 Q – Have we succeeded in the prosecution withdrawal of STRmix evidence?

A – We always recommend the review of STRmix evidence. Where we find the STRmix™ evidence lacking in robustness, we set out our opinion and reasoning accordingly. Our expert opinion in this regard has successfully resulted in the government laboratories amending evidence, capitulation and withdrawal of STRmix™ evidence.


 Q – Is STRmix™ used internationally?

A – STRmix is being used by 78 organisations in America, including FBI laboratories, and the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Additionally, it is also used by 26 laboratories in Canada, The United Kingdom, Ireland, Europe, Asia, the Middle East and the Caribbean. All state and territory labs on New Zealand and Australia currently use STRmix™.



Ms Jae Gerhard

Forensic Scientist

Starting out as a Forensic Scientist at the Australian Federal Police in 2002, Jae is an expert in body fluid detection and confirmation, bloodstain pattern analysis and DNA analysis and interpretation. With comprehensive experience in Australian Police Forensic Forensic Services, she’s also experienced in the reporting of DNA in familial relationships for Disaster Victim Identification (DVI) and parentage testing. Jae now provides impartial reviews of forensic biology evidence as an independent consultant and is currently completing a PhD at the Centre for Forensic Science at UTS where she’s an honorary associate.