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Case assessment and interpretation – application to a drugs supply case
Proceedings
Case assessment and interpretation - application to a drugs supply case G Booth, F Johnston and G Jackson Forensic Science Service, Chepstow Laboratory, Usk Road, Chepstow, Gwent NP6 6YE, United Kingdom Presented at The Forensic Science Society Autumn Meeting and AGM, Chorley, November 2001
In recent years the Forensic Science Service (FSS) has been engaged in developing a framework that provides a consistent, robust and balanced approach to forensic science casework. This framework is known as the Case Assessment and Interpretation (CAI) model [ l 4 ] . Here we attempt to show how the model may assist in resolving assessment and interpretational issues in a case involving the supply of controlled drugs. We will compare this with what might be considered to be the 'traditional' approach, demonstrate how important it is to be precise on the meaning of words used and to be clear on which part of the drugs 'supply chain' is being considered. Let us first look at our example 'case'. The submission details are as follows: Mr P is arrested in possession of two wraps Mr S is arrested in possession of 28 wraps Mr S will be charged with 'Supply'; Mr P with 'Possession'. The Police request which accompanies the submission is to 'compare the wraps from Mr P with those from Mr S'. Examination of the two items reveals that the packaging on both sets of wraps is shiny, catalogue paper and it is of 'similar' appearance but there are no physical fits. The drug in both sets of wraps is heroin. The powders 'match' on purity, GCMS and profiling. 'Traditionally', when reporting these findings we might use words along the lines of 'The packaging is similar but there are no physical fits between the pieces of paper '. 'There is no conclusive evidence to link the paper in the two items. ' 'The paper wraps are likely to be from the same catalogue, or from different catalogues of the same type'. 'The paper wraps were similar in appearance but I found no specific link between them.' 'The powders are likely to have came from the same batch but I am unable to say how large this batch would have been. ' Does the reporting of findings in this fashion provides maximum value, clarity and balance to the Criminal Justice System or is there a better way? Let us demonstrate how the principles of the CAI model may be applied to our drugs case example.
Firstly the model requires us to have: - clarity
on the framework of circumstances and the customer's
needs; - clarity on the issues that are being addressed by the scientist; -
appropriate data with which to inform scientific opinion.
So let us go back to the 'Police Officer' in the case and put some questions to him that will assist us in gaining further clarity on the circumstances of the case and his needs.
Q. What other information is there?
A. An exchange between Mr P and Mr S was observed. Mr S was arrested soon afterwards. Q. What does Mr P say?
A. He says that he bought the two packages from Mr S. Q. What does Mr S say?
A. He says that he bought his wraps locally for his own use and that he did not supply Mr P.
Q. What is the allegation? A. That Mr S supplied drugs to Mr P. Q. What is the real need?
A. To obtain evidence to support the possession and supply charges. We can translate the Police need into a list of possible questions we may consider addressing: - Did
Mr S supply the drugs?
- Did
the two wraps come from the 'group' of wraps in Mr S's possession?
-Were the two 'lots' of wraps produced by the same packaging 'operation'? - Did the wrappings around the two wraps come from the same 'source' as those around the 28? - Did
the drug in the two wraps came from the same 'batch' as the 28?
The above questions move progressively away from the key
63 The Forensic Science Society 2002 Key words Forensic science, drugs, case assessment, interpretation
science&justice Volume 42 No.2 (2002) I 23 - 125
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G Booth, F Johnston and G Jackson Case assessment and interpretation - application to a drugs supply case
point at issue insofar as they are addressing different points in the drugs supply chain. The use of words such as 'batch', 'source', 'operation' and 'lot' may be used to convey the idea of these different supply chain positions. It is therefore important that the scientist is clear in his or her own mind about the precise meanings given to these words and conveys them in any report. When evaluating the evidence presented to it, the court will be endeavouring to answer specific questions that will help it to reach a decision about the guilt or innocence of the defendant. Therefore the more closely the scientific evidence is aligned with one or more of these questions the more helpful it becomes. It seems unlikely that any scientist would feel comfortable with trying to address the offence level question - the ultimate issue for the jury! However in this case we will attempt to address a question very close to the 'supply' activity, i.e., - Did
the two wraps come from the 'group' of wraps in Mr S's possession?
The CAI model requires us to convert this question into a 'proposition' of the sort - The two wraps came from the group in Mr S's possession. This Prosecution proposition may be straightforward if based on a witness story or an allegation. However its formulation may be difficult if there are no witnesses or a specific allegation. The model also requires us to formulate an alternative, or Defence, proposition against which to weigh the evidence. The alternative proposition should be derived from the defendant's 'story'. It is important to note here that this is an idealised case in that we have information from both parties. In most cases such detail will not be available. If the defendant provides a 'no comment' interview it may be possible to construct a defence position around the assertion that he is completely innocent and has no association with drugs. However such defence positions may tend to 'maximise' the evidence against the defendant. At the other extreme constructing an 'explanation' for the findings will minimise the evidential significance of the findings. How to deal with cases in which the defendant makes 'no comment' is currently the subject of considerable debate. In drugs supply cases a critical piece of information for the construction of the alternative proposition may be whether the defendant admits to being a drugs user or not. It seems appropriate in this case to use the alternative proposition - The two wraps came from a different group from that in Mr S's possession. Note that the word 'group' is here being used to show that we are considering the proposition that the two quantities of tablets seized from the defendants had been together as a unit up to the point when they were separated as part of the alleged supply transaction. Contrast this with words such as 'batch' and 'source', which might be used to describe the drug or packaging manufacturing process.
2. the alternative were true. The likelihood ratio (LR), and hence the evidential strength is derived from the ratio of these two probabilities. In our case the likelihood ratio is expressed by, LR =
Pr(E) if the two wraps came from the group in Mr S's possession Pr(E) if the two wraps came from a different group from that in Mr S's possession
For the 'prosecution' proposition (numerator) we need to consider the probability of the packaging and contents of the two wraps having their particular features, ifthey had come from the group in Mr S's possession. In order to make such an assessment it would be useful to know something about the variation within the group of 28 wraps. We would expect there to be a high probability that the properties of the two wraps, if they had been taken at random from the original group of 30, would fall within this range. Without more analytical data it is not possible to be precise about this figure but it seems appropriate to assign a high probability (close to 1.) In order to address the 'defence' proposition (denominator) we need to consider the probability of the packaging and contents of the two wraps having their particular features if the two wraps came from a different group from that in Mr S' possession. To assess this likelihood we are considering the variation in heroin wraps in the locality of the alleged incident. Such data may be provided by specific surveys, the collection of drug intelligence data from routine casework or based upon the personal experience of the scientist - or a combination of these. For illustrative purposes let us assume that wraps of the type in this case account for 1 in 50 of seizures amongst our chosen data set. Therefore using the figures suggested above we obtain a likelihood ratio of about 50 (i.e. it is 50 times more likely that we would obtain the observed findings if the prosecution assertion were true than if the 'defence'assertion were true.) This may be translated (using an appropriate verbal scale - see Reference 4) into an expression of 'moderate support'for the assertion that the two wraps in Mr PS possession were formerly part of the group found in Mr S's possession rather than part of another group of wraps in the locality. Note that if the findings andlor data had been such that the LR generated was close to 1 then the evidence would not assist either way in resolving the issue at hand (the findings being equally likely if either proposition were true.) If the findings were more likely if the alternative proposition were true then the LR would be less than 1. The evidence would then provide greater support for the defence position, i.e. that the two wraps had come from a group other than that in the possession of the defendant, rather than the prosecution one.
The Scientist's role is now to consider questions of the type 'what is the probability of the observations [Pr(E)] $.?'
It is important to appreciate that the evaluation of the findings depends critically upon the pair of propositions that are being addressed. These in turn are based upon the background circumstances as supplied to the scientist. If these change then the findings will need to be re-evaluated.
1. the proposition were true, and
It can be seen that the specification of propositions, particularly
Page 124
science&justicc Volume 42 No.2 (2002) 123 - I 25
G Booth, F Johnston and G Jackson Case assessment and interpretation - application t o a drugs supply case
the defence one, is crucial in identifying the sort of data that may be required for interpretation. Current research work is developing this notion further in an attempt to develop a generic approach.
References 1
2
In summary we believe that the benefits of this model over the 'traditional' approach are that it encourages: -Consistency. Because the CAI framework can be taught, it provides a basis for consistency of approach by all scientists. -Robustness. The scientist is encouraged to carefully consider the factors such as background information and data that he or she is using to evaluate findings and to declare these in the report.
Cook R, Evett IW, Jackson G, Jones PJ, and Lambert JA. A model for case assessment and interpretation. Science &Justice 1998: 38: 151-156.
Cook R, Evett IW, Jackson G, Jones PJ and Lambert JA. A h~erarchyof propositions: deciding which level to address in casework. Science & Justice 1998; 38: 231-239.
3
IW Even, G Jackson and JA Lambert. More on the hierarchy of proposltlons: exploring the distinction between explanations and proposltlons.Sclence & Justice 2000: 40: 3-1 0.
4
IW Evett, G Jackson, JA Lambert, S McCrossan. The impact of the pr~nc~ples of evidence interpretation on the structure and content of statements. Science &Justice 2000: 40: 233-239.
- Balance. The model causes the scientist to consider both the
prosecution and defence position thereby providing a balanced assessment of the evidence. It means that phrases such as 'consistent with' should fade from the forensic scientist's assessment vocabulary. -Added value. By addressing the 'customer' requirement, tailoring the examination strategy and providing expert assessment. References The authors would like to acknowledge the assistance of colleagues within the drugs teams and Interpretation Research Group of the FSS in developing the ideas conveyed in this talk.
science&justice Volume 42 No.2 (2002) 123 - 125
Page 125
Case assessment and interpretation - application to a drugs supply case G Booth, F Johnston and G Jackson Forensic Science Service, Chepstow Laboratory, Usk Road, Chepstow, Gwent NP6 6YE, United Kingdom Presented at The Forensic Science Society Autumn Meeting and AGM, Chorley, November 2001
In recent years the Forensic Science Service (FSS) has been engaged in developing a framework that provides a consistent, robust and balanced approach to forensic science casework. This framework is known as the Case Assessment and Interpretation (CAI) model [ l 4 ] . Here we attempt to show how the model may assist in resolving assessment and interpretational issues in a case involving the supply of controlled drugs. We will compare this with what might be considered to be the 'traditional' approach, demonstrate how important it is to be precise on the meaning of words used and to be clear on which part of the drugs 'supply chain' is being considered. Let us first look at our example 'case'. The submission details are as follows: Mr P is arrested in possession of two wraps Mr S is arrested in possession of 28 wraps Mr S will be charged with 'Supply'; Mr P with 'Possession'. The Police request which accompanies the submission is to 'compare the wraps from Mr P with those from Mr S'. Examination of the two items reveals that the packaging on both sets of wraps is shiny, catalogue paper and it is of 'similar' appearance but there are no physical fits. The drug in both sets of wraps is heroin. The powders 'match' on purity, GCMS and profiling. 'Traditionally', when reporting these findings we might use words along the lines of 'The packaging is similar but there are no physical fits between the pieces of paper '. 'There is no conclusive evidence to link the paper in the two items. ' 'The paper wraps are likely to be from the same catalogue, or from different catalogues of the same type'. 'The paper wraps were similar in appearance but I found no specific link between them.' 'The powders are likely to have came from the same batch but I am unable to say how large this batch would have been. ' Does the reporting of findings in this fashion provides maximum value, clarity and balance to the Criminal Justice System or is there a better way? Let us demonstrate how the principles of the CAI model may be applied to our drugs case example.
Firstly the model requires us to have: - clarity
on the framework of circumstances and the customer's
needs; - clarity on the issues that are being addressed by the scientist; -
appropriate data with which to inform scientific opinion.
So let us go back to the 'Police Officer' in the case and put some questions to him that will assist us in gaining further clarity on the circumstances of the case and his needs.
Q. What other information is there?
A. An exchange between Mr P and Mr S was observed. Mr S was arrested soon afterwards. Q. What does Mr P say?
A. He says that he bought the two packages from Mr S. Q. What does Mr S say?
A. He says that he bought his wraps locally for his own use and that he did not supply Mr P.
Q. What is the allegation? A. That Mr S supplied drugs to Mr P. Q. What is the real need?
A. To obtain evidence to support the possession and supply charges. We can translate the Police need into a list of possible questions we may consider addressing: - Did
Mr S supply the drugs?
- Did
the two wraps come from the 'group' of wraps in Mr S's possession?
-Were the two 'lots' of wraps produced by the same packaging 'operation'? - Did the wrappings around the two wraps come from the same 'source' as those around the 28? - Did
the drug in the two wraps came from the same 'batch' as the 28?
The above questions move progressively away from the key
63 The Forensic Science Society 2002 Key words Forensic science, drugs, case assessment, interpretation
science&justice Volume 42 No.2 (2002) I 23 - 125
Page 123
G Booth, F Johnston and G Jackson Case assessment and interpretation - application to a drugs supply case
point at issue insofar as they are addressing different points in the drugs supply chain. The use of words such as 'batch', 'source', 'operation' and 'lot' may be used to convey the idea of these different supply chain positions. It is therefore important that the scientist is clear in his or her own mind about the precise meanings given to these words and conveys them in any report. When evaluating the evidence presented to it, the court will be endeavouring to answer specific questions that will help it to reach a decision about the guilt or innocence of the defendant. Therefore the more closely the scientific evidence is aligned with one or more of these questions the more helpful it becomes. It seems unlikely that any scientist would feel comfortable with trying to address the offence level question - the ultimate issue for the jury! However in this case we will attempt to address a question very close to the 'supply' activity, i.e., - Did
the two wraps come from the 'group' of wraps in Mr S's possession?
The CAI model requires us to convert this question into a 'proposition' of the sort - The two wraps came from the group in Mr S's possession. This Prosecution proposition may be straightforward if based on a witness story or an allegation. However its formulation may be difficult if there are no witnesses or a specific allegation. The model also requires us to formulate an alternative, or Defence, proposition against which to weigh the evidence. The alternative proposition should be derived from the defendant's 'story'. It is important to note here that this is an idealised case in that we have information from both parties. In most cases such detail will not be available. If the defendant provides a 'no comment' interview it may be possible to construct a defence position around the assertion that he is completely innocent and has no association with drugs. However such defence positions may tend to 'maximise' the evidence against the defendant. At the other extreme constructing an 'explanation' for the findings will minimise the evidential significance of the findings. How to deal with cases in which the defendant makes 'no comment' is currently the subject of considerable debate. In drugs supply cases a critical piece of information for the construction of the alternative proposition may be whether the defendant admits to being a drugs user or not. It seems appropriate in this case to use the alternative proposition - The two wraps came from a different group from that in Mr S's possession. Note that the word 'group' is here being used to show that we are considering the proposition that the two quantities of tablets seized from the defendants had been together as a unit up to the point when they were separated as part of the alleged supply transaction. Contrast this with words such as 'batch' and 'source', which might be used to describe the drug or packaging manufacturing process.
2. the alternative were true. The likelihood ratio (LR), and hence the evidential strength is derived from the ratio of these two probabilities. In our case the likelihood ratio is expressed by, LR =
Pr(E) if the two wraps came from the group in Mr S's possession Pr(E) if the two wraps came from a different group from that in Mr S's possession
For the 'prosecution' proposition (numerator) we need to consider the probability of the packaging and contents of the two wraps having their particular features, ifthey had come from the group in Mr S's possession. In order to make such an assessment it would be useful to know something about the variation within the group of 28 wraps. We would expect there to be a high probability that the properties of the two wraps, if they had been taken at random from the original group of 30, would fall within this range. Without more analytical data it is not possible to be precise about this figure but it seems appropriate to assign a high probability (close to 1.) In order to address the 'defence' proposition (denominator) we need to consider the probability of the packaging and contents of the two wraps having their particular features if the two wraps came from a different group from that in Mr S' possession. To assess this likelihood we are considering the variation in heroin wraps in the locality of the alleged incident. Such data may be provided by specific surveys, the collection of drug intelligence data from routine casework or based upon the personal experience of the scientist - or a combination of these. For illustrative purposes let us assume that wraps of the type in this case account for 1 in 50 of seizures amongst our chosen data set. Therefore using the figures suggested above we obtain a likelihood ratio of about 50 (i.e. it is 50 times more likely that we would obtain the observed findings if the prosecution assertion were true than if the 'defence'assertion were true.) This may be translated (using an appropriate verbal scale - see Reference 4) into an expression of 'moderate support'for the assertion that the two wraps in Mr PS possession were formerly part of the group found in Mr S's possession rather than part of another group of wraps in the locality. Note that if the findings andlor data had been such that the LR generated was close to 1 then the evidence would not assist either way in resolving the issue at hand (the findings being equally likely if either proposition were true.) If the findings were more likely if the alternative proposition were true then the LR would be less than 1. The evidence would then provide greater support for the defence position, i.e. that the two wraps had come from a group other than that in the possession of the defendant, rather than the prosecution one.
The Scientist's role is now to consider questions of the type 'what is the probability of the observations [Pr(E)] $.?'
It is important to appreciate that the evaluation of the findings depends critically upon the pair of propositions that are being addressed. These in turn are based upon the background circumstances as supplied to the scientist. If these change then the findings will need to be re-evaluated.
1. the proposition were true, and
It can be seen that the specification of propositions, particularly
Page 124
science&justicc Volume 42 No.2 (2002) 123 - I 25
G Booth, F Johnston and G Jackson Case assessment and interpretation - application t o a drugs supply case
the defence one, is crucial in identifying the sort of data that may be required for interpretation. Current research work is developing this notion further in an attempt to develop a generic approach.
References 1
2
In summary we believe that the benefits of this model over the 'traditional' approach are that it encourages: -Consistency. Because the CAI framework can be taught, it provides a basis for consistency of approach by all scientists. -Robustness. The scientist is encouraged to carefully consider the factors such as background information and data that he or she is using to evaluate findings and to declare these in the report.
Cook R, Evett IW, Jackson G, Jones PJ, and Lambert JA. A model for case assessment and interpretation. Science &Justice 1998: 38: 151-156.
Cook R, Evett IW, Jackson G, Jones PJ and Lambert JA. A h~erarchyof propositions: deciding which level to address in casework. Science & Justice 1998; 38: 231-239.
3
IW Even, G Jackson and JA Lambert. More on the hierarchy of proposltlons: exploring the distinction between explanations and proposltlons.Sclence & Justice 2000: 40: 3-1 0.
4
IW Evett, G Jackson, JA Lambert, S McCrossan. The impact of the pr~nc~ples of evidence interpretation on the structure and content of statements. Science &Justice 2000: 40: 233-239.
- Balance. The model causes the scientist to consider both the
prosecution and defence position thereby providing a balanced assessment of the evidence. It means that phrases such as 'consistent with' should fade from the forensic scientist's assessment vocabulary. -Added value. By addressing the 'customer' requirement, tailoring the examination strategy and providing expert assessment. References The authors would like to acknowledge the assistance of colleagues within the drugs teams and Interpretation Research Group of the FSS in developing the ideas conveyed in this talk.
science&justice Volume 42 No.2 (2002) 123 - 125
Page 125