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Critical Reasoning: Conclusion Weakening Questions

A low-calorie, low-fat, high-protein diet has been shown to assist in increasing longevity. The average life expectancy among the Jani community in Ambatia is significantly higher than it is among any other community in the country. The Jani community is located in a remote, secluded area, and its only contact with the outside world is a monthly delivery of groceries from the nearest town unless the weather is too bad for the journey. Members of the community grow most of their food, as well as goats and chickens, which they occasionally eat. This essentially imposes a low-calorie, low-fat, high protein diet. Clearly, these circumstances account for the longer lives of the Jani community members.     

Which of the following, if true, casts the most doubt on the above conclusion?

You're right!

[[snippet]]

This answer choice attacks the argument's assumption. Because we're talking about dietary factors, we must relate to the entire life of an individual. If a Jani member lived until 91, but only joined the community at age 74, then the diet has only influenced a certain fraction of that member's lifetime. This seriously weakens the idea that the diet causes the longevity.

Incorrect.

[[snippet]]

This answer choice neither weakens nor strengthens the argument's conclusion. Since the argument deals with the average life expectancy of the citizens of Ambatia in the Jani community and outside it, the total number of individuals in each group cannot affect the comparison.

Incorrect.

[[snippet]]

This answer choice neither weakens nor strengthens the argument's conclusion. The topic of medical expenses is irrelevant since the conclusion deals only with diet. This answer choice presents data that is true for both the Jani community and the rest of the citizens of Ambatia. Because we are comparing the two, stating something that is common to both does not help us in our task of weakening the conclusion.   

Incorrect.

[[snippet]]

This answer choice merely repeats one of the argument's premises by specifying the actual life expectancy ages. The second sentence of the argument already states that "the average  life expectancy among the Jani community in Ambatia is significantly higher than it is among any other community in the country."

Incorrect.

[[snippet]]

This answer choice neither weakens nor strengthens the argument's conclusion.

Since the argument deals with the average life expectancy in Country X in general and in the Jani community in particular, a single individual's age is not enough to cast doubt on the conclusion.

Many people who live in the Jani community joined it after the age of 40 or after retirement.
The Jani community has less than a thousand members.
In Ambatia, all medical expenses are paid for by the government, so that personal income does not affect the quality of health care a person receives.
The life expectancy in Ambatia is 76 years, compared to 84 years in the Jani community.
The oldest person in Country X is 97 years old, and she is not a member of the Jani community.