Self-study
Researchers in South Australia estimate changes in shark populations inhabiting local waters by monitoring what is termed the “catch per unit effort” (CPUE). ███ ████ ███ ███ ███████ ██ █████ ██ ███ ██████ ██ █████ ██████ ████ ██████████ █████████████ █████ █████ ███ ████ ███ ████ █████████ ██ ████ ███ ███ ███ ██ ███ ██████ █████ ████ ███ ████ ███ █ ██████████ ███████ ██ █████ ███ ████████ ██████ █████████ ██████████ ███ ██████████ ██ ████ ███████ ██ ███ ██████ ██████ █████ █████████ ████ ██ ██ █████████████ ███ ████ ██████
Weakening The Argument: Bros Slangin’ Nets
Let’s build this up backward. The stem says we need to weaken the argument. Well, here’s the argument:
Premise: The CPUE has been constant since 1973.
________
Conclusion: The shark population has been constant since 1973.
To weaken this argument, we’re brainstorming possible worlds in which the shark population has changed even though the CPUE stayed the same. For that, we need to know wtf the CPUE is. I think of it like this:
One dudebro. One hour. One km of net. How many sharks can he catch?
The CPUE establishes a standard unit of trying, then measures how many sharks you get when you try that hard. If dudebro catches a ton of sharks, the CPUE is a big number and researchers figure that means there’s a bunch of sharks swimming around. If dudebro catches very few sharks, the CPUE is a small number and researchers figure that means there’s only a few sharks swimming around.
Now we can frame our brainstorming more specifically:
We need to imagine possible worlds in which dudebro has been catching the same number of sharks, but the shark population has changed.
Side Note: A number of notable domain restrictions should hover in the back of your mind. All the stimulus’ claims are about South Australia. They’re about one specific species of shark. These restrictions mean info about other regions or other species is irrelevant. The CPUE measures sharks per boat and per hour and per km of net. This means suggestions that more boats have been fishing for more hours or using bigger nets are irrelevant.
16.
Which one of the following, ██ █████ ████ █████████ ███████ ███ █████████
a
The waters around █████ █████████ ███ ███ ████ ████ ██ ███ █████ █████ ████ ██████████ ███████ ██ █████ ██ ██████
(A) would be relevant to a conclusion about the global population of this shark species, but this argument is only about the shark population in South Australia.
Even if this species were entirely extinct everywhere else, that wouldn’t affect the shark population in South Australia.
b
The sharks that ███ ███ ████ ██████████ ██ █████ ███ █████ ████ ████ ██ ██████ ██ ███ ████ ████ ██ █████ ████ █████ ████ ███ ███ ███████ ███ ████ █████ ████ ████ █████
(B) says fishers (sharksters?) prefer to catch stationary sharks, which doesn’t threaten the CPUE as a measure of shark population.
Like maybe (B) suggests our particular species of shark is a stationary shark, so sharksters love hunting them and their population has declined significantly. The researchers’ take is that if this were true, it would show up in the CPUE – it would be harder for dudebro to catch as many sharks as he used to. (B) gives us no tools to challenge that narrative.
c
A significant threat ██ █████ ████████████ ██ ████████ ██ ██████████ █████ ████████ ██ █████████████ ████████████ ████ ███████ ████ ████████ ██████ ██ ████ ████████ ███ █████ █████
(C) introduces another potential threat to the shark population aside from intentional hunting, but this doesn’t undermine the CPUE as a measure of the shark population.
Let’s say tons of sharks are getting caught up in tuna nets and the population has declined significantly. The researchers’ take is that if this were true, it would show up in the CPUE – it would be harder for dudebro to catch as many sharks as he used to. (C) gives us no tools to challenge that narrative.
d
Most of the ██████ ████████ ██ ███████ █████ ███████████ █████ ███ ███████ ██ ██████ ████ ███ ██ █████ ███ ███ ███ ██████ ██ ██████████ ███████
(D) draws a distinction between number of sharks and pounds of sharks. It would be relevant if the CPUE measured one thing and the conclusion was about the other. But both the CPUE and the conclusion are about the number of sharks.
If the CPUE were about tonnage and not number, then (D) would weaken the argument by suggesting commercial boats have been catching lots of tiny sharks, leaving behind a smaller population of big sharks. That’s probably the lens through which (D) is most tempting. But again, the CPUE is about number.
e
Since 1980 commercial █████████████ █████ ████ ████ █████████████ ██████████ █████████ ████ ███████ ████ ██ ██████ ██████ ████ ███████ █████████
Whereas the stimulus hypothesizes that the reason CPUE has remained constant is because the shark population has remained constant, (E) suggests an alternative hypothesis: perhaps the shark population has been in decline but we've simultaneously gotten better at catching sharks.
It could be that back in 1973 commercial boats were out there dragging their nets around randomly. They caught a few sharks through sheer luck because the sharks were so plentiful. Now there are way fewer sharks, but the commercial boats plop themselves right on top of them before casting their nets.
Same CPUE; smaller shark population.