Which one of me following, if true, would most seriously undermine the validity of the results of the experiment disc...

Ravi on May 21, 2019

@Shiyi-Zhang,

Great question. Let's take a look at (A) and (B).

(A) says, "The live birds all came from different titmouse flocks."

(A) would definitely strengthen the results of the experiment in the
last paragraph by getting rid of an alternative cause. It's possible
that the titmice used in the experiment all were from the same very
timid flock of titmice. (A) rules this out, which allows us to draw
our conclusion. Thus, (A) does not undermine the validity of the
results, so it's out.

(B) says, "The physical characteristics of the stuffed dummies varied
in ways other than just breast stripe width."

We know from the passage that the experiment discussed in the final
paragraph dealt with titmice with varying stripe sizes. The titmice
with larger stripes scared the other titmice away, whereas the titmice
with the smaller stripes didn't scare the birds away. In examining
these results, we know that the larger stripes caused the birds with
smaller stripes to act submissively. In order to weaken a causal
argument like this, one thing we can do is to provide an alternative
cause. (B) provides us with an alternative cause that suggests that
the stripe might not have been the factor that influenced the birds to
be scared or aggressive. It could have been some other characteristic.
This weakens the validity of the experiment, so (B) is the correct
answer choice.

Does this make sense? Let us know if you have any more questions!