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Critical Reasoning Assumptions Tutorial

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Types of Assumptions

Assumptions remove the gap between the evidence and the conclusion, based on the way they do it assumptions can be divided into several categories.

1. Causal Assumptions establish a cause-effect relationship between the evidence and the conclusion.

Evidence: After GEOCOM implemented a new marketing strategy five years ago, the company’s profits have increased by 15 percent.

Conclusion: The increase in profits can be credited solely to the new marketing strategy.

Assumption: Over the last five years GEOCOM did not make any changes besides implementing the new marketing strategy.

As you can see the evidence only shows that implementation of the new strategy and the increase in profits have coincided in time, and it could be true that something else and not the new strategy could have caused the increase in profits. The assumption eliminates this possibility and strengthens the conclusion drawn in the argument. To check whether this is the necessary assumption, try to negate it: If over the last five years GEOCOM did make some changes besides the implementation of the new marketing strategy, then it could be true that those changes have contributed to the increased profits, and in this case the conclusion that increased profits can be credited solely to the new marketing strategy is not reasonable.

Note that such arguments assume that there is only one possible cause of the effect, especially if there are words like: solely, only, fully, etc.

Often a correct answer to this type of assumption question will prove that the cause and effect relationship was not reversed. Consider the following example:

Evidence: People with low blood pressure generally have lower secretion of hormone X than do people with normal or high blood pressure.

Conclusion: Low blood pressure causes decreased secretion of hormone X.

Assumption: Low level of hormone X secretion does not cause low blood pressure.

It must be assumed that low secretion of X does not cause low blood pressure because otherwise the conclusion would be illogical and it could be true that people have low blood pressure because of low hormone X secretion and not otherwise

2. Assumptions which show similarity between concepts in evidence and conclusion.

Evidence: Investment companies are losing money now.

Conclusion: Company X has financial difficulties now.

Assumption: Company X is an investment company.

This example is much like our Daisy-cow example before. The evidence is about investment companies and the conclusion is about Company X. For such a conclusion to be logical it must be true that Company X is an investment company.

3. Assumptions which prove the validity of evidence.

Evidence: Mr. Dillinger is allergic to animals and is going to use a new medication called Kirtanol.

Conclusion: Mr. Dillinger will soon be able to keep animals at home.

Assumption: Kirtanol is effective against allergic reaction to animals.

Here the evidence only shows that Mr. Dillinger is using a new medication and the conclusion states that he will soon be relieved from his problem. This is only true if we assume that the new medication is an effective means against allergic reaction to animals.

Download Critical-Reasoning Cheat Sheet

Contributed by Hemang Jain

Try the following Assumption Questions

1) A recent report determined that although only 3 percent

2) Graduates of medical schools are interested in practical work as practicing physicians

3) Since the new publisher took control, a news magazine’s covers have featured only models

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