Some Error Rates Associated with Bloodstain Pattern … drop of human blood from 35 cm onto ceramic...
Transcript of Some Error Rates Associated with Bloodstain Pattern … drop of human blood from 35 cm onto ceramic...
INTRODUCTION
MATERIALS AND METHODS
RESULTS
CONCLUSIONS
REFERENCES
• Bloodstain pa,ern analysis allows for the es3ma3on of Area of Origin.
• Area of Origin relies on the calcula3on of angle of impact.
• Li,le research has been conducted to quan3tate error rates associated with angle of impact calcula3ons (Willis et al., 2001).
• Our goal was to inves3gate the effect of impact angle and surface texture on error rate.
• Error rates increased as angle of impact increased (Table 1)
• Error rates < 1.2°
• Only one angle of impact measured accurately
• Surface texture had a significant (P < 0.05) effect on error rates at 10° and 50° but not at 90° (Table 1)
Blood • Human blood (1.07g/mL ± 0.003g/mL) drawn intravenously into preserva3ve free vials
Surfaces • Three surfaces of contras3ng texture used (ceramic 3le, porcelain 3le, mirror)
Experimental Design • Tiles placed at three angles (10°, 50°, 90°) • Blood dropped (0.03mL ± 0.001mL) onto 3les from Pasteur pipe,e at height of 35 cm
• Each set of impact stains measured by four individuals varying in levels of exper3se ranging from novice to expert
• Experiment replicated ten 3mes
Sta7s7cal Analyses • Data did not meet the assump3ons set forth for normality and homogeneity of variance as determined by the Kolmogorov-‐Smirnov test and Levene’s test, respec3vely
• Comparison of means conducted using the Kruskall-‐Wallis H and Mann-‐Whitney U sta3s3cs
• Imposing ± 3.5° on 50° and 90° impact stains and ± 1.9° on 10° impact stains made our AOI calcula3ons accurate.
• These error rates are smaller than those currently used by bloodstain pa,ern analysts (Allard et al., 2010).
• This allows for more precise es3mates of area of origin.
Table 1. Angle of impact estimated by four analysts ranging in experience following the drop of human blood from 35 cm onto ceramic tile, mirror tile, or porcelain tile within 3 minutes of blood leaving the body. Letters denote a significant (P < 0.05) difference between surface within angle of impact. Standard errors are presented in parentheses where n = 10.
Angles of Impact
Surfaces 10° 50° 90°
Ceramic 11.6° (0.2°)ab 53.5° (0.7°)a 88.3° (0.9°)a
Mirror 11.9° (0.1°)a 50.3° (0.6°)b 86.5° (1.2°)a
Porcelain 11.3° (0.1°)b 48.8° (0.5°)b 87.8° (1.1°)a
Figure 1: 10° impact stains on porcelain tile
Figure 2: 90° impact stains on ceramic tile
• Allard KL, Kish PE, Gestring BJ (2010) Evalua3ng the validity of angle of impact/incidence determina3ons made from very small bloodstains. Proceedings of the 62nd Annual Mee3ng of the American Academy of Forensic Sciences, 16:116.
• Willis, C., Piranian, A.K., Donaggio, J.R., Barne,, R.J., & Rowe, W.F. (2001). Errors in the es3ma3on of the distance of fall and angles of impact blood drops. Forensic Science Interna.onal, 123, 1-‐4.
Some Error Rates Associated with Bloodstain Pattern Analysis
Kaitlin Leslie1, Taylor Lowis1, Larry E. Barksdale2, David O. Carter1 1Department of Entomology, University of Nebraska- Lincoln, 68583-0816
2Lincoln Police Department, Lincoln, Nebraska