Logical Reasoning

PT109 · S1 · Q17 Expert witness: Ten times, and

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Expert witness: Ten times, and in controlled circumstances, a single drop of the defendant’s blood was allowed to fall onto the fabric.

Conclusion (I conclude)

A single drop of the defendant's blood stains much less than 9.5 cm2 of the fabric.

Evidence

Ten times, a single drop of the defendant's blood was allowed to fall onto the fabric, and all ten times the stained area was between 4.5 and 4.8 cm2

Evaluate

This question specifically asks us to weaken the value of the evidence, so we're not necessarily doing our usual thinking of,

The correct answer might just give us a way to argue that the evidence is sketchy (bad methodology or something like that).

For example, maybe dropping blood straight down out of an eyedropper creates less of a stained area than if you propel blood at the fabric at an angle.

Maybe the cold / hot / humidity conditions of the testing room make the stained area smaller than in would be in other conditions.

Goal

It's hard to anticipate answers when they go after the evidence. The correct answer should make us feel very differently about the logical force of the evidence, either by making it seem like we can't really trust the evidence we have or making it seem like weaker support for the conclusion than it currently is.

17.

Which one of the following, if true, most undermines the value of the evidence for the expert witness's conclusion?

  1. If similar results had been

    Too Weak

    This answer informs us that the evidence is not currently 100% maximally strong, since there's room for it to be stronger. But the fact that something could be stronger does not make it weaker. If I brew some strong coffee, and then my wife asks, "Wow. What could weaken the strength of this coffee?", it doesn't count to say "Hey, I could've made it even stronger!"

    3% picked this

  2. Expert witnesses have sometimes been

    Too Weak: sometimes

    This answer creates a possibility that this expert witness might be among those that fudge their data. (Fudging data = stretching it / fabricating it so that it says what you want it to say) Is this data fudged? Maybe, maybe not. The strength of this answer is sometimes, so it's only telling us that there was "at least one expert witness who once fudged her data". That creates a sliver of doubt about this witness, but no more than that.

    11% picked this

  3. Correct

    In an eleventh test drop

    Why this is right

    #1 this highlights the potential variability of this blood dropping test and makes us think that the current ten-drop evidence was insufficient to give us a clear picture. The ten times they previously dropped blood were all between 4.5 and 4.8, and so the witness is saying, "It looks like the witness's blood is always gonna be in that range". The fact that the eleventh drop was 9.3, twice as large as the other ten trials, shows that there is a lot of variability, and so to know the range of possibility we need to do more than ten drops. #2 this actually hurts the conclusion. The conclusion is saying "a single drop stains much less than 9.5", but 9.3 is not much less than 9.5; it's only a tiny, tiny bit less.

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

    65% picked this

  4. Another person’s blood was substituted,

    Strengthens, if anything

    This answer shows that it was theoretically possible to stain above 9.5 in this testing environment. If the testing environment wasn't preventing 9.5 from happening, then we'd have to assume it's the nature of the defendant's blood that was doing so. Perhaps the defendant's blood is viscous and doesn't spread as much, whereas this other person is on blood thinners and so stains a much wider area. We can't use this answer to say, "You see? Your test was messed up", because it seems to prove the opposite. The test isn't the problem; another person's blood was able to stain something resembling 9.5 cm.

    18% picked this

  5. Not all expert witnesses are

    Too Weak: not all

    This answer creates a possibility that an expert witness might not really be an authority in the field of blood dropping. Does that mean that this witness isn't an authority? Maybe, maybe not. The strength of this answer is not all, so it's only telling us that there is "at least one expert witness who is not an authority in their field." That creates a sliver of doubt about this witness, but no more than that.

    3% picked this

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