nederlands forensisch instituut observed and expected numbers of (partially) randomly matching...
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Nederlands Forensisch Instituut
www.forensischinstituut.nl
Observed and expected numbers of (partially) randomly matching profiles in the Dutch DNA database, and in international DNA searches
Marjan SjerpsKees van der BeekAte Kloosterman
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Sept 11 2009 (adventitious) matches in databases
The Dutch DNA offender database
• National DNA database of the Netherlands at August 2009:• 86,929 persons• 40,170 stains• 19,876 hits (stain-person)• 4,628 hits (stain-stain)
• Numbers are updated every month at www.DNAsporen.nl
• We have used the offender database to empirically test our random match probability (RMP) calculations
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Sept 11 2009 (adventitious) matches in databases
How good are the RMP estimates we report?
• Method is given in :• Weir (2004) Matching and partially matching DNA
profiles, J Forensic Sci, Vol. 49, 1009-1014 • Weir (2007) The rarity of DNA profiles, The Annals of
Applied Statistics Vol. 1, No. 2, 358–370
• Compare two 10-locus DNA profiles • Full match: at 10 loci both alleles match• Partial match: e.g. at 8 loci both alleles match
but at 2 loci only 1 allele matches• Compare the expected number of partial matches
to the observed number in a large database
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Sept 11 2009 (adventitious) matches in databases
Applying Weir (2004, 2007) to NL data
• Database contains 73,895 DNA profiles from suspects/offenders at January 16 2009
• Database pre-processing:• 773 matches were found (aliasses and duplicates?);
only single copy retained • 1578 partial profiles were removed: only full SGM+
profiles (10 loci) retained; • 71,544 10-locus profiles of different persons left
• Consider all pairs of profiles (>2.5 billion pairs)• Observe number of pairs matching at e.g. 8 loci
and compare to expected number
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Sept 11 2009 (adventitious) matches in databases
Observed versus expected numbers of partially matching profiles
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Sept 11 2009 (adventitious) matches in databases
Relatives?
Observed versus expected numbers of partially matching profiles
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Sept 11 2009 (adventitious) matches in databases
How representative are reference databases?
• We compared allele frequencies of offender database (n=71,544) to reference database (n=231) of Dutch Caucasians
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Sept 11 2009 (adventitious) matches in databases
0
0.05
0.1
0.15
0.2
0.25
0.3
9 11 12 13 14 15 15.2 16 17 18 19 20
D3 allele
NL offender(n=71544)NL reference(n=231)
Allele frequencies: offender & reference database
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Sept 11 2009 (adventitious) matches in databases
Allele frequencies: offender & reference database
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
15.2 17
18.2 19
19.2
20.2
21.2
22.1
22.3
23.2 24
24.2 25 26 27 28 29 30 31
32.2
44.2
46.2
FGA allele
NL offender(n=71544)NL reference(n=231)
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Sept 11 2009 (adventitious) matches in databases
Conclusion 1
• Investigating offender databases provides important empirical information about the validity of our RMP estimates
• The Dutch data provide:• empirical support for the RMPs that are
routinely reported (theta=0.01)• empirical support that theta is close to 0• empirical support for the assumption that our
reference dataset of Dutch Caucasians is sufficiently representative
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Sept 11 2009 (adventitious) matches in databases
International database search
• The Netherlands searches the databases of Germany, Austria, Slovenia, Luxembourg and Spain every day for matches with stains (Prüm treaty)
• Other European databases will be available in the future
• These searches produce huge numbers of pairwise comparisons of DNA profiles
• Two kinds of DNA matches:• “assisting” matches : the matching profiles are indeed from
the same person and hence support the investigation• “adventitious” matches: the matching profiles are from two
different persons
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Sept 11 2009 (adventitious) matches in databases
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Sept 11 2009 (adventitious) matches in databases
Two examples of international search
• We report on data of two searches:• The search performed when starting the
exchange with Germany (July 2008)• A search in the UK database with a selection
of crime stains (February 2008)
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Sept 11 2009 (adventitious) matches in databases
NL-DE exchange: 6 and 7 locus matches
• The Netherlands has searched the German DNA-database (524,782 persons and 123,862 stains) with 25,249 DNA-profiles from stains
• 16 billion (1.6 x 1010) pairs of profiles were compared• Most of the comparisons have 7 loci in common, sometimes 6 loci
Nr of loci in common
Estimated nr of comparisons
RMP (theta=0)1 in…
Expected nr of adventitious matches
Observed nr of matches
7 12 billion 600 million
20 941
6 3 billion 50 million 61 291
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Sept 11 2009 (adventitious) matches in databases
• Expected nr of “assisting” matches = Observed - expected adventitious = 1151
• But we expect about 81 adventitious matches • Therefore, as standard procedure matches are
upgraded by typing more loci (SE33 or SGM+) before any personal data are exchanged (following recommendation 5 in ENFSI report)
• Practical difficulty: Germany has the policy of immediate destruction of DNA reference samples after analysis, which precludes any additional testing of the reference samples
NL-DE exchange: 6 and 7 locus matches
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Sept 11 2009 (adventitious) matches in databases
NL-UK exchange:6 and 10 locus matches
• The Netherlands has searched the UK DNA-database (4.8 million reference profiles) with 2159 DNA-profiles from stains of serious unsolved crimes
• Some of the NL stain profiles were SGM typed (6 loci)
Nr of loci
Nr of NL profiles
RMP (theta=0)1 in…
Database size UK
Expected nr of adventitious matches
Observed nr of matches
6 602 50 million 4.8 million
58 28
10 1575 2500 billion
4.8 million
0.003 17
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Sept 11 2009 (adventitious) matches in databases
• 28 SGM matches (6 loci) were upgraded to SGM+ (10 loci), only 5 still matched
• So the other 23 SGM matches were adventitious matches• 5+17=22 SGM+ matches in total; no adventitious
matches expected• So about 22 matches can be used to assist unsolved
serious cases• Hence, searching with 6 loci in this example was very
useful: upgrading only 28 profiles resulted in 5 SGM+ matches
• However when not upgraded this kind of search generates lots of adventitious matches
NL-UK exchange:6 and 10 locus matches
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Sept 11 2009 (adventitious) matches in databases
Conclusion 2
• Searching with partial profiles is very useful but produces adventitious matches
• Therefore upgrading, if possible, is necessary before reporting (rec.5 ENFSI report)
• Upgrading not always possible due to practical or legal limitations
• Newer kits and upgrading procedures will reduce the number of adventitious matches considerably
• Meantime, searching with mixtures and partial profiles in (inter)national databases without upgrading will produce adventitious matches
• Database annual reports should report on matches that were reported but later turned out to be adventitious
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Sept 11 2009 (adventitious) matches in databases
Reporting database matches
• Chakraborty and Ge, Forensic Science Communications July 2009:
• “Thus it can be reasoned that cold-hit cases in which the suspect is identified in the absence of valid alibis for not having access to the crime scene, a DNA match can and should be quantified by RMP alone without any additional changes”
• We have argued (Meester and Sjerps 2003,2004) that RMP alone is misleading
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Sept 11 2009 (adventitious) matches in databases
ENFSI document on DNA database management
ENFSI-recommendation 22• A DNA-database match report of a crime scene
related DNA-profile with a person should be informative and apart from the usual indication of the evidential value of the match (RMP) it should also contain a warning indicating the possibility of finding adventitious matches (as mentioned in recommendation 21) and its implication that the match should be considered together with other information.
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Sept 11 2009 (adventitious) matches in databases
Box in NL database match reports
• …As the number of DNA-profiles in the database increases, also the probability increases of observing a match with a person who is not the stain donor. The profile of this person matches “coincidentally” with the profile of the trace.
• …One has to take this into account especially when a DNA-database match is observed involving an incomplete or mixture profile….
• …For assessing the possibility of an adventitious match it is important whether there is other tactical or technical evidence that associate this person with the crime.
• …More information is available in “The essentials of forensic DNA testing” [NFI practical professional annex for jurists, also
available in English, contact [email protected]]
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Sept 11 2009 (adventitious) matches in databases
Conclusion 3
• It is misleading to report a DNA database match by only mentioning a RMP
• Report should include a warning (recommendation 21 and 22 ENFSI report)
• NFI and custodian of NL national database include “point of attention”-box in their reports