CAI Flashcards
what 3 things does the criminal justice system expect of forensic science from the police and courts?
1 - intelligence - timely information to link or exclude suspect (DNA, fingerprints, mobile phones, CCTV)
2 - corroborative evidence - to support charge/prosecution
3 - timely scientific support through all stages of CJS
what 4 things does the criminal justice system expect of forensic science from politicians and society
1 - rapid DNA results, quality assured casework, clear statements, good testimony
2 - an impact on crime, crime reduction & crime prevention
3 - partnership projects in serious & volume crime
4 - an objective measure of ‘value’
what 2 things happened in 1990 surrounding the origination of CAI
1 - forensic science services (FSS) became an Executive Agency of Home Office
2 - direct charging of police for forensic services
- development of ‘forensic products’
- costs no longer ‘invisible’ to police
what 5 things was the FSS doing in 1999
1 - experiencing significant case backlogs & long turnaround times
2 - constantly re-negotiating delivery dates
3 - drowning in unnecessary examinations & lengthy (manual) analytical processes
4 - struggling even to meet some court dates
5 - failing to meet the needs of police & CJS clients
what were 2 typical submission requests in 1999
- “… forensicate (sic) these items”
- “.. I need a ‘full forensic on these items”
what did the 1988 local government act of finance statue cause
1 - ‘best value’ requirement - local government review of services
2 - police authorities were not exempt
3 - forensic costs no longer invisible
4 - cost review = better resource allocation = best value
what is best value not equal to?
best value ≠cheapest option
what are the 4 objectives of the CAI model
1 - improve decision making and resource allocation
2 - improve relationships and develop partnerships
3 - provide value (defined by client agency)
4 - service the needs of the criminal justice system
what are the 2 elements of CAI
- assessment
- interpretation
define the assessment element of CAI
how do we manage, prioritise and triage the demand
define the interpretation element of CAI
how do we report results in a logical, quantitative way
what are the 3 phases of the CAI model
1 - customer requirement, case information, customer needs
2 - case pre-assessment, determine forensic strategy, identify propositions
- this is all pre-submission
3 - service delivery, forensic examination, evaluates outcomes, interpretation and report
what is the hierarchy of propositions and give an example of this
- source (has semen originated from suspect)
- activity (has suspect had sexual intercourse with complainant)
- offence (did suspect rape the complainant)
what framework (5 things) does the CAI model provide for the investigator
1 - ensures requirements are clearly articulated
2 - assists in setting forensic examination strategy
3 - assists decision making
4 - provides staged information (Investigative or Evaluative phases)
5 - enhances value (cost/time vs. usefulness = value)
what framework (4 things) does the CAI model provide for the criminal justice system
1 - ensures a balanced (unbiased) approach
2 - ensures impartiality and transparency
3 - allows discussion of strength of evidence
4 - adds value (not to be confused with cost)
what is the 3 steps of the forensic process
1 - crime scene examination (investigation)
2 - laboratory examination (evaluation)
3 - court (adjudication)
what 5 things does CAI require
1 - excellent communication between all key parties
2 - clear understanding of each other’s requirements
3 - agreement of forensic examination strategy
4 - understanding of expected outcomes
5 - exploration of staged approach where possible
what is the equation for the posterior odds for the court
posterior odds = likelihood ratio x prior odds
(the court = the scientist x other info)
Pr(GIE)/Pr(ḠIE) = Pr(EIG)/Pr(EIḠ) x Pr(G)/Pr(Ḡ)
where
E = evidence
G = guilty
Ḡ = innocent
Pr = probability of
what framework (6 things) does the CAI model provide for the forensic scientist
1 - ensures clear understanding of case information and issues
2 - leads to an agreed forensic examination strategy
3 - defines propositions for testing
4 - enables staged examination and reporting
5 - is underpinned by sound statistical theory
6 - adds value to the scientist’s role
What are the principles of Bayes Theorem
- evaluation
- what scientist must do/be able to do
- likelihood ratio determines?
- evaluation must be carried out within a framework of circumstances (I or information)
- the scientist must make clear their understanding of the case circumstances relevant to the evaluation (conditioning information)
- scientist must consider their observations in the light of prosecution Hp and defence Hd propositions (propositions must be clearly stated and subject to change)
- scientist must consider the probability of their findings E if the prosecution/defence proposition were true
- relative magnitude of these two probabilities (LR) determines the assistance provided by their findings in weighing the two propositions against each other
What propositions are addressed?
What happens if expert can only address one proposition?
- address at least one pair of propositions based on prosecution issue and one based on alternative/defence issue
- if a reasonable alternative cannot be identified, the expert may address only the one proposition but making it clear they cannot evaluate the strength of the evidence
What is the likelihood ratio?
- probability of the evidence given the prosecution hypothesis and background info divided by the probability of the evidence given the defence hypothesis and the info
- probability of the evidence if the prosecution version of events is true divided by the probability of the evidence if the defence version of events is true - all taking into account relevant contextual information
What is conditional probability?
- the likelihood of an event occurring given that another event has already occurred
What is independent odds?
- the likelihood of two events occurring independently of each other
- the occurrence of one event does not effect the probability of the other event occurring
What was the case of Sir Roy Meadow?
- he used independent odds to work out the likelihood of another infant death when he should have used conditional odds
- Sally Clark was incorrectly prosecuted for the murder of her two children as they didn’t use independent odds
- Roy Meadows statistical error was connected with a failure to appreciate the base rate (family with one cot death more likely to have another, not increase like he did because they were well off)
Woman alleges she was raped by male acquaintance after a party
- she was examined 5 hours after
- items submitted:
- internal/external vaginal and thigh swabs
- pants worn subsequent to alleged offence
- DNA ref sample from injured party
- DNA ref sample from suspect
- examine the items from the complainant for semen and determine if it has come from suspect
- what is your examination strategy
- have you got all the information you need
- scientific understanding of issues:
- did suspect have sexual intercourse with the complainant or did he ejaculate between her legs?
- ACITIVTY (level 2)
- customer request
- examine the items from the complainant for semen and determine if it has come from the suspect
- SOURCE (level 1)
- propositions:
- Hp = suspect had sexual intercourse with complainant as alleged
- Hd = suspect didn’t have sexual intercourse with complainant but ejaculated between her thighs (consensual act)
- LR is Pr (E I Hp and I)/Pr (E I Hd and I)
- possible outcomes (semen, some semen and lots of semen)
- examine internal swab for semen
- examination of panties not seen as useful
- what about examination of external vaginal and thigh swabs
What do values of LR indicate?
LR > 1 supports Hp
LR = 1 is neutral
LR < 1 supports Hd
Death of Adela Rebelo
- was found dead in her flat in Jersey in 2017 (died from compression to neck caused by ligature - daughters tights)
- husband was arrested and charged for her murder
- Deputy Viscount Advocate Mark Harris recorded open conclusions (said homicide, suicide, accidental self-strangulation were all possibilities)
- Hp - husband used tights to asphyxiate her
- Hd - husband found her in shared house and removed leggings after death
- evidence was large amount of transfer to Alfredo on the leggings
What is the prosecutors fallacy?
- the probability of the proposition depending on the evidence instead of the probability of the evidence given the proposition
- when the prosecutor incorrectly interprets the probability of evidence given the defendants guilt as the probability of guilt given the evidence
What is transposing the conditional?
- sometimes called the prosecutors fallacy
What is Bayes Theorem given two events A and B?
- Bayes theorem is given by:
- P (A I B) = P (B I A) x P (A) / P (B)
- where P (A I B) = probability of condition when event A is occurring while event B has already occurred
Doheny and Adams (1996)
- appellant was convicted of rape and sentenced to 8 years of imprisonment
- case relied on DNA evidence (semen stains found on clothing and blood group information from semen stain and blood group information from Doheny)
- in court, asked about match probability of the DNA evidence combined with blood group evidence and his response was ‘taking them all into account, i calculated the chance of finding all of those bands and conventional blood groups to be about 1 in 40 million’
- this probability is the probability of finding that set of DNA and blood type in general population
- he then asked the prosecution: ‘the likelihood of it being anybody other than Alan Doheny?” he answered, “is about 1 in 40 million”
- here the expert has been asked what is the probability that Doheny is not the source of the semen, given the matching DNA profiles and blood types BUT his answer reverses the conditional
- prosecutors fallacy/transposing the conditional
the probability of the proposition, given the evidence makes this error?
- prosecutors fallacy
Establishing the prior odds calculates the?
- base rate
Jill Dando
- a well-known TV personality was shot dead on doorstep of London home
- no witnesses, obvious motives
- single shot to back of head, no firearm found, cartridge case and bullet recovered
- Barry George was interviewed a year later
- a single particle of GSR was found inside one of the pockets of a coat found in his flat and the composition o the GSR particle was indistinguishable from GSR particles at crime scene
- two questions must ask yourself
- are you sure it is particle of GSR
- are you sure particle was not deposited on coat as a result of adventitious contamination
- then must decide if prosecution has made sure this particle was deposited on the coat other than adventitiously
- if sure you can exclude innocent contamination, then take this matter into account
- if not sure prosecution has proved its case then discard evidence
- all experts agree there is a possibility contamination could have occurred adventitiously
- crown says particle overwhelmingly supports crown’s case
- defence says it is incredible this can still be used of incriminating evidence as such particles are shed rapidly and could not survive on coat that long
base rate
- number of cases/population
When is their no need to examine all the evidence that has been submitted
- reducing cost of case and time taken
- no comment interview = everything approach
- take a staged approach and only use what is needed
