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NIST Inter Laboratory Study 2013

Friday, 24 July 2015

Recently Barry Scheck spoke at some length at a meeting on the New York State Commission on Forensic Science [1].  Amongst other matters he stated that a certain software was the only software that gets the NIST Mix 13 [2] exercise correct.  We are a little disappointed at Mr Scheck’s statements at this important meeting.  His statement about NIST Mix 13 is untrue.

We did not participate in MIX13 at the time that the study was undertaken.  We have subsequently analysed the five cases within the study.  The STRmix™ results are summarised in the table below.

 

Likelihood ratio for person of interest [3]

Outcome

Case

a

b

c

d

1

2.60x1019

NA

NA

NA

Correct

2

2.12x1016

465,000

822,000

2.35x10-7

Correct

3

21,000

0

NA

NA

Correct

4

1.71x1018

NA

NA

NA

Correct

5

3 person solution

295,000

50,000

0

NA

Correct

4 person solution

1,400,000

1,000,000

0

NA

Correct

 

The outcome is considered ‘correct’ based on a high likelihood ratio for true contributors and a low one for non-contributors. 

Case 5 was challenging.  The mixture is actually a 1:1:1:1 mix of individuals a and b and two other individuals selected to maximise allelic overlap (the genotypes follow).  0.5ng of each was amplified (total 2ng). 

 

Suspect 05A

Suspect 05B

"not tested"

"not tested"

AMEL

X,Y

X,Y

X,Y

X,Y

CSF1PO

12,12

12,13

11,11

11,12

D13S317

11,13

8,8

11,12

11,11

D16S539

11,12

11,11

11,13

11,12

D18S51

13,15

15,15

13,15

17,17

D19S433

13,14

14,15

12,14

14,14

D21S11

30,31

30,31.2

31.2,32.2

31,31

D2S1338

17,23

17,23

18,20

18,23

D3S1358

16,17

16,16

16,17

17,17

D5S818

11,13

11,13

11,13

11,12

D7S820

8,10

9,10

10,10

8,11

D8S1179

10,15

14,14

10,10

12,14

FGA

22,24

21,21

21,21

21,22

TH01

7,7

7,7

6,9.3

6,9.3

TPOX

8,9

11,11

8,8

8,11

vWA

15,16

17,18

16,17

17,17

 

The epg follows.

 epg

The allelic overlap and 1:1:1:1 ratio creates a perfectly viable three person solution.  The comparison of the peak fits for the 3 and the actual 4 person solutions is shown below.  The three person solution is a closer fit by least squares.

Locus

Allele

Observed heights (rfu)

Expected Three Donor Solution

Expected Four Donor Solution

D8S1179

9

57

69

69

10

1008

1086

1082

12

314

295

303

13

67

65

65

14

1105

1039

1043

15

387

365

358

 

D21S11

29

62

46

46

30

933

791

783

31

937

1023

970

31.2

690

688

749

32.2

336

368

364

 

D7S820

8

626

532

532

9

333

356

351

10

1118

1138

1137

11

212

230

236

 

CSF1PO

11

1000

1013

965

12

1230

1155

1212

13

314

316

310

 

D3S1358

15

115

125

125

16

2085

2078

2072

17

1801

1797

1804

 

TH01

6

1394

1328

1221

7

2338

2328

2313

9.3

1012

951

1073

 

D13S317

8

1223

1216

1210

10

111

141

143

11

2346

2249

2274

12

614

655

649

13

662

616

603

 

D16S539

10

141

180

173

11

2904

2883

2784

12

956

957

1068

13

590

571

566

 

D2S1338

16

57

53

53

17

1007

892

882

18

816

849

773

20

355

342

421

22

78

75

76

23

1102

1180

1184

 

D19S433

12

423

417

504

13

653

630

610

14

2358

2394

2237

15

417

386

470

 

vWA

15

595

550

545

16

1002

1015

1085

17

1947

1971

1820

18

421

398

484

 

TPOX

8

1889

1741

1666

9

441

349

423

11

1149

1203

1208

 

D18S51

12

56

57

56

13

924

894

881

14

125

113

112

15

1800

1765

1759

16

61

46

47

17

630

721

741

 

D5S818

10

87

111

111

11

1742

1770

1770

12

486

460

470

13

1400

1374

1363

 

FGA

20

119

120

120

21

1995

1919

1927

22

697

712

712

24

359

394

386

 

Looking at the genotypes, if one considers, say, CSF1P0, the four contributors are 12,12 and 12,13 and 11,11 and 11,12.  There is an equivalent solution as a 2:1:1 mix of 11,12 and 11,12 and 12,13.  This is true for all loci. 

STRmix requires the assignment of the number of contributors.  With no prior information, we would assign case 5 as a three person mixture.  STRmix™ gives mixture ratios 2:1:1 for this solution. 

If we assume the presence of reference a, as some software programmes do, we force the four person solution in STRmix and obtain the mixture proportions 1:1:1:1.  This is a valid assumption under Hp and we do not argue with the software programmes that do this.  However it would be wrong to make this assumption under Hd and the most parsimonious Hd solution is three contributors.   The only obvious way to decide that this is a four person mixture is to assume the presence of one of the persons of interest (POI): reference a, which we were not prepared to do.

However, whether assuming three or four contributors all POI have been correctly assigned valid likelihood ratios and included or excluded as contributors to the mixture correctly.

 

 

[1] June 19, 2015 meeting available here http://www.criminaljustice.ny.gov/pio/openmeetings.htm (external link)

[2] http://www.cstl.nist.gov/strbase/interlab/MIX13.htm (external link)

[3] LRs were calculated using the NIST Caucasian allele frequencies

 

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