1. Marjan Sjerps Kees van der Beek Ate Kloosterman
Observed and expected numbers of (partially) randomly matching profiles in the Dutch DNA database, and in international DNA searches
Marjan Sjerps
Kees van der Beek
Ate Kloosterman

2. 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
We have used the offender database to empirically test our random match probability (RMP) calculations
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3. 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,
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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4. Applying Weir (2004, 2007) to NL data
Database contains 73,895 DNA profiles from suspects/offenders at January
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, 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
Sept
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5. Observed versus expected numbers of partially matching profiles
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6. Observed versus expected numbers of partially matching profiles
Relatives?
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7. How representative are reference databases?
We compared allele frequencies of offender database (n=71,544) to reference database (n=231) of Dutch Caucasians
Sept
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8. Allele frequencies: offender & reference database
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9. Allele frequencies: offender & reference database
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10. 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
Sept
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11. 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
Sept
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12. Sept
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13. 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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14. 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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15. NL-DE exchange: 6 and 7 locus matches
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
Sept
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16. 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
0.003 17
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17. NL-UK exchange: 6 and 10 locus matches
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
Sept
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18. 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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19. 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
Sept
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20. ENFSI document on DNA database management
ENFSI-recommendation 21
DNA-database managers should be aware of the possibility of adventitious matches and be able to calculate their expected numbers for the matches they report. When reporting a DNA-database match, a warning should be included indicating the factors that increase the possibility of finding an adventitious match (size of the database, number of searches, mixed and partial profiles/ random match probability, presence of family members)
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21. 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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22. 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
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23. 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
Sept
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