Summarize Argument
The author concludes that prehistoric rock paintings on small islands off the coast of Norway do not represent a description of the painters’ diets. This is because the author believes the painters needed to eat the sea animals in the waters around Norway in order to travel to the islands, and no paintings clearly show those sea animals.
Notable Assumptions
The author assumes that the paintings don’t depict sea creatures. (Just because they don’t unambiguously depict sea creatures doesn’t imply that they don’t show sea creatures.) The author also assumes that the painters needed to cross the waters to get to the islands. (Maybe the painters originated on the islands?) The author also assumes that the painters needed to eat sea creatures.
A
Once on these islands, the cave painters hunted and ate land animals.
This shows that the painters’ current diets did not need to include sea creatures. So the lack of sea creatures in paintings does not prove that the paintings don’t represent the painters’ current diets.
B
Parts of the cave paintings on the islands did not survive the centuries.
This raises the possibility that sea creatures might have appeared in paintings that are no longer around. So, the cave paintings could have been a description of the current diets of the painters, even if the paintings that remain no longer depict sea creatures.
C
The cave paintings that were discovered on the islands depicted many land animals.
The author never denied that the paintings could show land animals. The author’s position is that we’d expect to see sea creatures, too. So, the presence of land animals in the paintings doesn’t undermine the author’s argument.
D
Those who did the cave paintings that were discovered on the islands had unusually advanced techniques of preserving meats.
This raises the possibility that the painters did not need to eat sea creatures, contrary to the author’s assumption. The painters might have been able to travel to the islands while eating preserved meat from land animals.
E
The cave paintings on the islands were done by the original inhabitants of the islands who ate the meat of land animals.
This shows that the author’s assumption that the painters needed to cross the islands and eat sea creatures is wrong. The painters could have been done by inhabitants of the islands who may not have needed to eat sea creatures.
Summarize Argument
The statistician claims there’s reason to be skeptical about the magazine’s claim that North Americans are more concerned with finances than politics. Why? Because a question on the survey was biased and because subscribers to the magazine might poorly represent North Americans in general.
Notable Assumptions
The statistician assumes conclusions based on a survey with a self-selecting sample and a biased question should be received skeptically. He assumes the survey question he quotes is biased in a way that could affect the conclusion based on the survey’s results.
A
The credibility of the magazine has been called into question on a number of occasions.
This is another reason to view the survey’s results with skepticism. It suggests the magazine has been accused of being careless or insincere, which doesn’t prove the magazine’s conclusion wrong, but does give a reason to be skeptical of it.
B
The conclusions drawn in most magazine surveys have eventually been disproved.
This is another reason to view the magazine’s conclusion with skepticism. It suggests magazine surveys in general are unreliable, which doesn’t prove this magazine’s conclusion is false, but gives a reason to be skeptical about it.
C
Other surveys suggest that North Americans are just as concerned about politics as they are about finances.
This introduces new evidence that conflicts with the magazine’s conclusion, giving more reason to doubt that conclusion.
D
There is reason to be skeptical about the results of surveys that are biased and unrepresentative.
This makes concrete the statistician’s assumption that a survey with an unrepresentative sample and a biased question should be viewed skeptically.
E
Other surveys suggest that North Americans are concerned not only with politics and finances, but also with social issues.
This is irrelevant. Neither the magazine nor the statistician assumes politics and personal finance are the only two issues North Americans care about. Concern about a third issue gives no reason to doubt the magazine’s conclusion, which compares interest in the two issues only.
Note: At 1:30, I made a mistake calling the rule a bi-conditional. If it were a bi-conditional, then /Y→S. That’s just not true. /Y doesn't trigger S.
The correct interpretation of the rule is just a straight up "not both" rule, S→/Y.
Thanks to the 7Sagers who pointed this out.