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Critical Reasoning - Structure of Assumption vs Inference

We need to discuss an important distinction between assumptions and inferences. Most of you will be able to explain the difference between an assumption and an inference but some questions will still surprise you. After all, both assumptions and inferences deal with the same elements in the argument. The way they are worded makes all the difference.


In simple words, when you have enough data given and you can infer something from it without doubt, it is called an inference/conclusion.
When you have author’s opinion (conclusion of the argument) and you need something to be true for the opinion to hold, that is an assumption.


Let us explain with a simple example.


All A are B.
All B are C.

You can conclude that: All A are C. This must be true. It is a conclusion.


If you conclude that ‘All C are A’ (your opinion, not necessarily a fact), you are assuming that A, B and C overlap i.e. they all have exactly the same elements.

Look again:


Argument 1:


All A are B.
All B are C.

Conclusion: All A are C.


Argument 2:


All A are B.
All B are C.

Conclusion: All C are A.

Assumption: A, B and C overlap.


The conclusion of argument 2 will not hold if the assumption is negated.


If you are wondering why we are emphasizing it again and again even though it looks really simple, here is an official question that might help you understand the reasons for our misgivings. We will give you the argument but not the question stem. We will also give you the correct answer. You will need to decide whether the question stem asks for a conclusion or an assumption.


Question: Among the more effective kinds of publicity that publishers can get for a new book is to have excerpts of it published in a high-circulation magazine soon before the book is published. The benefits of such excerption include not only a sure increase in sales but also a fee paid by the magazine to the book’s publisher.

(A) The number of people for whom seeing an excerpt of a book in a magazine provides an adequate substitute for reading the whole book is smaller than the number for whom the excerpt stimulates a desire to read the book.

(B) Because the financial advantage of excerpting a new book in a magazine usually accrues to the book’s publisher, magazine editors are unwilling to publish excerpts from new books.

(C) In calculating the total number of copies that a book has sold, publishers include sales of copies of magazines that featured an excerpt of the book.

(D) The effectiveness of having excerpts of a book published in a magazine, measured in terms of increased sales of a book, is proportional to the circulation of the magazine in which the excerpts are published.

(E) Books that are suitable for excerpting in high-circulation magazines sell more copies than book that are not suitable for excerpting.


The correct answer is (A). What is the question?


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