Letter to the Editor — The Bias Snowball and the Bias Cascade Effects: Two Distinct Biases that May Impact Forensic Decision Making

Abstract

Seven different potential sources of bias are presented in Fig. 1 (for their full descriptions and examples see (1)). They include innate sources relating to the mere fact that we are human (the very bottom of the taxonomy), general sources that emerge from the experience, training and environment in which forensic examiners operate, and also the specifics of the case being investigated (the top of the taxonomy that includes the improper use of reference material as "targets" that drive the forensic comparison-suspect-driven bias-i.e., working back-ward from the suspect/target to the evidence, rather than the other way around; see (1,2) for details). Official bodies, such as the UK Forensic Regulator (3) and the US National Commission on Forensic Science (4), have now acknowledged the potential of cognitive bias in forensic work.However, the question remains as to the mechanisms of how such sources translate to actually cause bias. Here, we should distinguish between the bias cascade and the bias snowball effects.Consider, for example, that in some jurisdictions, the CSI personnel who collect evidence from the crime scene are the same people who also do the forensic work back in the labora-tory. In such cases, the analysis, evaluations, interpretations, and conclusions at the forensic laboratory may be influenced by irrelevant contextual information that examiners may have been exposed to at the crime scene. It is not always simple and self-evident what information is relevant and what is irrelevant, but clearly there are many pieces of information that are totally irrelevant to the forensic examiner (see the National Commis-sion on Forensic Science document "Ensuring that forensic analysis is based upon task-relevant information" (4)). The bias cascade effect is when bias arises as a result of irrelevant infor-mation cascading from one stage to another, e.g., from the ini-tial evidence collection to the evaluation and interpretation of the evidence.The bias cascade effect can take many forms, all sharing the characteristic that irrelevant information in Time 1 (e.g., during evidence collection at the crime scene) cascades to Time 2 (e.g., when the evidence is interpreted). Countering such bias cascade can be achieved by controlling the information flow between the different stages of the forensic investigation (2,5,6).First, it is best to have different people involved at the various stages of the forensic investigation. For example, it is ill-advised that those who collect evidence at the crime scene (who are exposed to a variety of contextual information, much of it needed to do their job) will be the same people who examine and interpret the evidence back at the forensic laboratory

A cultural change to enable improved decision-making in forensic science: A six phased approach

“…For example, evidence disclosed to the police as a result of one isolated piece of forensic science analysis could begin to suggest a particular scenario, which could lead to tunnel vision within the police investigation due to the high motivation to identify and apprehend a suspect [75,76]. Conversely, the opinion of the police (which could also potentially be led by tunnel vision [75,77], cascade bias (the impact of contextual information at one stage in the reconstruction process impacting decisions being made at a later stage) and snowball bias (when a number of pieces of contextual information interact and have an increasing 'snowballing' impact on a decision that is made) [66], driven by other aspects of the investigation [78,79]) could impact upon the communication strategy of the police to forensic scientists. For example, when dealing with ambiguous evidence from a complex crime scene, a scenario could occur where the police and/or the prosecution have a strong belief of the guilt of a suspect.…”

“…These decisions are often made under the highly pressured and emotive conditions which are considered to be potential fuels for subconscious influences in decision-making [98]. Given the environment in which these decisions are made and the potential for snowball effect and cascade bias within decisionmaking later in the forensic reconstruction process [66], it is important that empirical studies are extended to cover a wider remit of forensic reconstruction decisions, encompassing the whole forensic reconstruction process [21]. Equally, the impact of the presentation of findings of the forensic science process on the judgements, decisions, and interpretations of the investigative team, key stakeholders within the legal domain, and judges and juries should be comprehensively explored, both in terms of the existing mechanisms of reporting forensic outcomes and also in relation to the application of new techniques and technology (for example the use of 3D technology [99] or virtual reality reconstruction within the courtroom [100,101], or the application of novel approaches to data collection [102], analysis [103], or interpretation [104,105]).…”

“…It is not always simple and self-evident what information is relevant and irrelevant in order to decide what to prioritise. This has the potential to create interpretive difficulties at a later stage if the decisions about what to collect at the crime scene have been influenced by misleading information or irrelevant context [36,66]. Similarly, decisions made at the crime scene or laboratory to process certain finger marks rather than others have the potential to be crucial in the interpretation and subsequent intelligence gathering stages [67].…”

Evaluation of forensic genetics findings given activity level propositions: A review

“…To assign probabilities for factors such as DNA transfer, persistence and prevalence, detailed information is needed on relevant, though not necessarily all, case circumstances. Much has been written on the cognitive processes that may lead to bias [109][110][111], the sources of biasing information [112] and potential avenues to minimize the risks of cognitive bias in (amongst others, DNA) evidence interpretation in forensic science [113][114][115][116]. These papers deal with the risk when the evaluation is based on sub-source level propositions, but they are equally (or possibly more) relevant for evaluation with propositions at other propositional levels.…”

Diagnosing Crime and Diagnosing Disease‐II: Visual Pattern Perception and Diagnostic Accuracy

“…For example, departmental procedures which limit the use of blinding can lead to potential bias “snowball” effects, and interexpert bias effects . With the snowball effect, bias increases as irrelevant information from a variety of sources is integrated and influences each other .…”