1. Fibers and Probability
Interpreting the Evidence

2. The fibers have been collected and studied
How is the evidence used in a probative manner?

3. Chapter 6
The chance of finding a particular color/type combination among a random population of foreign fibers is very low
The chance of a collective of these fibers being present is even lower especially if synthetic fibers are involved.
The occurrence of a group of fibers on a surface that match those from a particular textile source shows strong evidence of contact with that source
The chance of finding differences between potentially matching target fibers recovered from a particular surface will increase proportionately to the number of comparative tests used to exam them (up to a point)
Kendall/Hunt

4. Fundamental questions
Chapter 6
Fundamental questions
1. What is the probability that the association
was due to coincidence?
2. What is the probability that the association
was due to examiner error?
3. What is the probability that there is an
alternative explanation - such as
secondary transfer, contamination or
deliberate planting of evidence?
Kendall/Hunt

5. Chapter 6
Auto Fiber Study… • 200 carpet samples were collected from 200 randomly selected vehicles from a 2000 vehicle junkyard in Northern Virginia in • Purpose: to conduct pair-wise comparisons of all collected samples. How many associations are there? Can a meaningful coincidence probability be obtained? • 200 samples compared with each other = comparisons forming a database
Kendall/Hunt

7. Sample characteristics
• 89.5% nylon • 10% polyester • .5% polypropylene – 91% trilobal (various shapes), 9% round But most individual fibers have very little dye

8. Database classes
Color – gray, brown, colorless, other… Cross section – regular, irregular, round and Michelin Man Delusterant – yes, no Is there a good way to included absorbance info into database?

12. Colorless Michelin Man – 32 samples

14. Actual matches
20/19900 = Probability of a coincidental match is approximately 1/1000 (conservative estimate) Base on microscopy and spectrophotometry

15. How “class”: Fibers can be Probative

17. More sampling techniques….
School administrators wish to know student attendance at a football game. They give out 50 bright yellow caps to students randomly selected as they enter the stadium. At the end of the game, as students are leaving the stadium, spotters observe 2 yellow hats in a selection of 25 , 1 cap among 35 students, and 4 hats among 40 students. How many students were at the game?

19. Chapter 6 .
Kendall/Hunt