Self-study
Support Five years ago, during the first North American outbreak of the cattle disease CXC, the death rate from the disease was 5 percent of all reported cases, whereas today the corresponding figure is over 18 percent. ██ ██ ██████ ██████████ ████ ██████ █████ ████ █ ██████ ███ ███ █████████ ██ ██████████
Structure: Phenomenon-Hypothesis
The stimulus reports a phenomenon: when CXC first broke out five years ago, the death rate among cattle was 5% of reported cases, and now it is over 18%. The stimulus therefore hypothesizes that CXC has become more virulent: i.e., the cause of this increase is that the disease itself has become more deadly.
Weakening: Notable Assumptions
The argument suggests a causal explanation for the increase in reported death rates from CXC among cattle. This relies on at least two key assumptions. The first assumption is that the increase in reported death rates reflects an increase in actual death rates, and is not just due to changes in reporting: maybe the death rate from CXC was closer to 18% all along, but many of the cattle who died due to CXC five years ago just weren't identified. Second, even if there was an actual increase in death rates, the hypothesis assumes that this increase was caused by CXC itself becoming more deadly, and not by some other factor that made cattle more vulnerable to CXC — for instance, some other disease that weakened the cattle ahead of the most recent CXC outbreak.
A good way to weaken this argument would be to target either of these assumptions and thus provide an alternative explanation for the apparent increase in deaths due to CXC — whether by suggesting that the apparent statistical increase is just due to changes in reporting, or by suggesting that there is another causal factor at play, instead of CXC itself becoming more virulent.
13.
Which one of the following, ██ █████ ████ █████████████ ███████ ███ █████████
a
Many recent cattle ██████ ████ ████ ████████ ████ ██████ ██ ███ ████ ████ ██████████ ██████████ ██ ███████ ███████ ████ ██████ ███ ████████ ██ ████
This is a tricky answer choice, because it looks a lot like one of the weakening strategies we discussed for this stimulus. But it's really important to think about the implications of this answer choice. (A) says that many recent cattle deaths caused by CXC have instead been attributed to another disease. This is talking about recent deaths, not deaths five years ago — and what it suggests is that the recent figure, 18%, is actually too low, because it doesn't account for all the deaths actually caused by CXC.
To weaken the argument, we would want an answer choice that explains away the statistical increase, whether by suggesting that the 5% figure from five years ago was too low, or the 18% figure from more recently is too high. (A) does neither. By suggesting that the actual recent figure is even higher than the 18% figure provided, far from providing an alternative explanation for the apparent increase in the death rate, (A) makes that increase seem even larger. So (A), if anything, strengthens the argument — it doesn't weaken it.
b
During the first █████ ████████ ████████ ██ ███ ████████ ████ ██ ███ ██████ ████████ ██ ████ ████ ██████ ██ ███ ████ ████████ ███ ██ █████ ███████
Again, remember that to "explain away" the apparent increase in the death rate by appealing to inaccurate statistics, we need to either make the 5% figure from five years ago seem too low, or the recent 18% figure seem too high. (A) didn't work because it made the 18% figure seem too low, and (B) also doesn't work, because it makes the 5% figure seem too high: if many of the deaths attributed to CXC five years ago were due to other causes, then the actual rate of deaths caused by CXC was even lower than 5%. Like (A), far from explaining away the apparent increase, this makes the increase seem even larger, and doesn't weaken the argument.
c
An inoculation program ███████ ███ ███ ████████ █████ █████ ██████████ ███████ ██████ ███████████ ██ ██ ██ ███████ █████████ ██ ██████████ ███████ █████ ██ ███ ████████
Incorrect. If this program was begun so "recently" that it has not yet had an effect, then it is irrelevant to the argument — but if we assume there was enough time for this inoculation program to take effect, we would expect death rates to go down, not up, given the efficacy rate of the inoculation. So, if anything, (C) strengthens the argument. If death rates went up despite the high efficacy of the inoculation program, this might lend support to the idea that CXC has become even more virulent.
d
Since the first █████████ ███████ ████ ███████ ██ █████ ████ █████ ██ ███ ███ ██ ██████ ██████ ████ ██ █████████████ ██ ████████████
Correct. If farmers are no longer reporting mild cases of CXC, then it makes sense that the death rate among reported cases of CXC has gone up: the reported cases now entirely consist of more serious cases of CXC, which are more likely to lead to death. So CXC hasn't necessarily become more deadly — it's just the "pool" of reported cases that has changed. This answer choice effectively weakens the argument.
e
Cattle that have ██████████ ███ ████████ ███ ██████ ████████ ███ ███████ █ ██████ █████
Irrelevant. We're interested in changes in the number of cattle that have contracted CXC and died, not in cattle that have survived and won't get it again.