#17 - Expert witness: Ten times, and in controlled

[HowardQ]

Hi,

I have a question about this problem. I thought B is a better answer than C, because fudge the data obviously undermine any statistical claim he makes. C, on the other hand, suggest 11th trail, which is not strong evidence compared to the previous 10 trials, and neither does it counter the conclusion. I don't understand why C is the right answer. Could someone help me out.

Thanks,

[Dave Killoran]

Hi Howard,

With answer choice (B), there's no evidence at all that the data was fudged. And, (B) references the prosecution's case, but we have no idea of this is an expert witness for the defense or the prosecution. We are no where near having this statement weaken the argument because there are so many holes in it.

With (C), why wouldn't an 11th test of the same blood be relevant? If 10 trials produced one result and then the 11th produced a totally different result, that calls into question the preceding 10. Sure, it's slightly less than 9.5, but look at the conclusion of the witness: "I conclude that a single drop of the defendant’s blood stains much less than 9.5 cm2 of the fabric" (italics added for emphasis). Answer choice (C) clearly undermines the conclusion as stated above.

Here's a rough analogy of what this looks like:

• Baseball Commentator: I've watch this player bat 10 times and he has never hit more than a single. Thus, I conclude he never hits anything more than singles.
  
  Answer choice (C): In his next at bat, the player hit a triple.

As seen above, (C) definitely weakens the argument, and it's a powerful weakener

Thanks!

[HowardQ]

Hey Dave,

Thanks for the answer, for your example I would agree more on the C stating that "in his next bat, he almost hit a double." I realized my mistake tho because I had a flaw in the reasoning attacking the personal character of the witness instead of the reasoning he uses.

[nickp18]

Hi PS Team!

I chose D on this question and was wondering if you could tell me if this is why I am incorrect. D is incorrect because it is referring to another person's blood and the expert witness specifically concluded something about only the defendant's blood?

Not only that, this is confirmatory evidence that the expert witness' conclusion is true about the defendant's blood only being able to spread this far?

Thanks,
Nick

[Rachael Wilkenfeld]

Exactly, Nick! We look to the conclusion to determine the scope of the argument. Here the conclusion is limited to the defendant's blood, so we only want to focus on the results using the defendant's blood. Answer choice (D) is talking about other people's blood and would not have a significant impact on the conclusion of this argument.

Excellent work!

[mab9178]

Hi,

Is it flawed to conclude that D strengthens the argument on the grounds that the fact that a drop of blood from a person, other than the defendant, staining the fabric in such markedly different size suggests that different people stain the fabric differently thereby giving credence the experiment suggesting something specific about the defendant?

I am not saying D makes the experiment tilt the trial in favor or against the defendant; just that D suggests that there is weight to this type of experiment: because another person blood drop stained the fabric so differently that the defendant did, it stands to reason that the size of a stain from a drop of blood suggests something that could exculpate or inculpate the defendant.

And if the experiment is revealing, then the argument is strengthened; correct?


Thank You
Mazen

[Adam Tyson]

I don't think so, Mazen. The conclusion is about the defendant's blood only, so whatever happens with someone else's blood would have no impact on the argument. It doesn't help to show that someone else's blood does something different, because the argument is not that the defendant's blood is somehow different from that of others.

[nicizle]

I'm a bit iffy on your analogy here, because the word "never" is very strong and makes your example weakener much more clear and foolproof. If the stimulus' argument had said "I conclude that a single drop of the defendants blood always stains much less than 9.5 cm," then I'd have picked C as a definite weakener.

Nonetheless, I picked answer choice A, as I felt the sample size of 10 drops was too small to make a conclusive statement. Can you explain why A is wrong, and why C is right? I didn't see how a single instance of a 9.2 cm blood drop was a substantive weakener when the first 10 drops were 4.5-4.8 cm. I'm not really seeing how C is a better answer, as it could just be an anomaly. I thought this type of occasional variance is normal in data sets.

[Dave Killoran]

Well, I said it was a rough analogy, but the first "never" in my example aligns with the example in the stimulus, which says "in all ten cases." That is the same as "never close to 9.5."

The second "never," in the conclusion, mirrors the "I conclude" language used in the stimulus. If you'd like, change my example from "never hits" to "doesn't hit." I'm trying to get across an easy-to-understand example that captures the idea behind what LSAC is thinking, and this analogy certainly works to show that and the "never" isn't a problem in either place. Conveniently, we know this is what LSAC is thinking because LSAC says this is the right answer

With (A), LSAC would ask how this weakens the argument. Sure this shows that the evidence would be strengthened if there were more tests, but how does that by itself weaken what the witness said? Simply saying you could have made it stronger doesn't by itself weaken the position because the evidence could've been strong enough already.

By the way, it's your concern over the small data set that actually makes (C) a good answer. 10 wasn't enough to prove anything, and the eleventh showed that those 10 trials didn't capture the range of outcomes very well.

Thanks!

[saiffshaikhh@gmail.com]

I am having trouble identifying why C, is the right answer. Sure, it says much less than 9.5 cm2 but 9.3 cm2 is arguably still much less than 9.5 cm2. I figured D would've been right because when controlling for the same circumstances, another persons blood draws a greater stained area. Not sure how that wouldn't weaken the experiment.