You don’t know what you don’t know

But does it matter? Or is everything inconclusive?

Disclaimer

• The opinions or assertions contained herein are the private views of the author and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense

• In fact, they may not even be the same views as those of my Technical Leader or my Boss, and they are both up on stage with me

Disclaimer (continued)

• To be honest, some things I may say today aren’t even my real opinions

• I’ve been known to play Devil’s Advocate just to make things interesting

• (In other words, sometimes I stir the pot just for the fun of it)

What is this profile? (ID amp)

This profile is…

1 2 3 4 5 6 7

14% 14% 14% 14%14%14%14%1. Not interpretable2. Single source female3. Single source male4. 2 person mixture5. 3 person mixture6. Cannot determine #

of contributors7. A “no-brainer”

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What about this one? (Y amp)

This profile is…

1 2 3 4 5 6 7

14% 14% 14% 14%14%14%14%1. Not interpretable2. Single source 3. 2 person mixture4. 3 person mixture5. Cannot determine # of

contributors6. A “no-brainer”7. It would be OK, but 11

at Y-GATA…CountdownCountdown

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It’s the same sample!

• The first one is an Identifiler amp of Victim’s underwear.

• The second one is a Yfiler amp of the exact same extract.

• 1 uL taken for ID amp (then diluted), followed by ~3 uL taken for Y amp from the same extract tube.

• (Other samples in the case show an Identifiler match from V evidence to S reference)

Yfiler profile of panties

Suspect Yfiler profile

Victim Identifiler profile

Suspect Identifiler profile

S

V

Alleles in common for V and S

S

D21

30 11 12 11* 14 8 12* 23

D7

S

V

CSF

S

V

S

V

D13*

D19S

V

TPOX

V

D5*S

V

FGAS

V

Now what?

• What does this mean?• Does this change your assessment of the ID

profile from V underwear?– You know there is a male in the sample– That male shares some alleles with the female– Did the enzyme sort through the shared alleles

and only amplify DNA that came from female cells?

What do you think about original ID profile?

1 2 3 4

25% 25%25%25%1. Cannot interpret2. Single source female3. Mixture

CONSISTENT with 24. Mixture AT LEAST 2

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What’s the point?

• You never know what you don’t know• In other words, you can never be 100% certain

of anything• Is this bad?

– Uh oh, is all data inconclusive?– Should we do CPI stat on every sample?

• # of contributors irrelevant• Easy to explain…• … but that whole stochastic drop out problem

Here’s the point

• You must draw a line in the sand everytime you make an interpretation– Validation studies– Interpretation guidelines– Your experience

• If it walks like a duck, talks like a duck, swims like a duck…. Might as well call it a duck.

What do I say?

• I still interpret that profile as a single source female.

• I have to go with what I see.– My validation studies– Interpretation guidelines/protocols – My experience– Plus, my tech reviewer agrees or report doesn’t go

out.

Take a stand on the stand

• If it’s consistent with a two person mixture, go ahead and interpret it as such.

• If it appears to be a three person mixture, treat it that way.

• State your opinion, and be confident.– Validation studies and mixture guidelines/protocols.– Your experience. – Your technical review process.

Should we do this?

• Section 3.6.5 – SWGDAM

• Define how you determine # of contributors in your protocols

• State it in your report

3.6.5. Because assumptions regarding the origin of evidence or the number of contributors to a mixture can impact comparisons, the laboratory should establish guidelines for documenting any assumptions that are made when formulating conclusions.

Wording

• We used to routinely say…– A mixture of DNA profiles from at least two

individuals was obtained from the...

• Now as much as possible we say…– A mixture of DNA profiles consistent with

originating from two individuals was obtained from the…

But what if…

• How do you handle it if challenged in court about the number of contributors?

• Here’s what I do: (Not telling you to do this)– Explain why I say 2 people (or 3)– Explain the difference between “consistent with 2

people” and “a mixture of 2 people”– If I keep getting challenged….

Why do I bring this up?

• The interpretations that you’ll see during this workshop rely on being able to determine the number of contributors

• The CPI stat is pretty much the only thing to use when you don’t know how many contributors

• The CPI stat has some limitations that affect how you can interpret a sample

These nifty “clickers”

• There are no right or wrong answers• Answer as best you can the way your lab

would do things• We can all learn how our lab compares to

other labs• No one will know how you answered

Did you look at the examples we emailed or watched the videos?

1 2 3 4

25% 25%25%25%

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1. What email?2. I looked at them but…3. I applied my lab’s

interpretation to them4. I was going to do it on

the plane, but there was this movie….

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