Scientific reasoning

Examine the conclusion in each of these passages. Explain why the conclusion is not well supported by the observations. See if you can suggest additional observations which would support the relevant conclusion.

  1. An IQ test is given to a large group of people. People over 60 generally have lower IQ scores. It is concluded that intelligence declines with age.
  2. A weight-loss clinic surveys the clients who completed a slimming program. 69.5% of those surveyed are very satisfied with the program. The clinic put out an advertisment saying that 70% of their clients are very satisfied with the services of the clinic.


The following classification of fallacies is proposed by Stephen Carey:

  1. Fallacies of irrelevance
  2. Fallacies of twisting the facts
  3. Fallacies of misusing language
  4. Fallacies of bad analogy
  5. Fallacies of bad inductive reasoning

Call this system of classification the "Carey system". Compare the Carey system with the 4i system presented in the course. See if you can identify different types of examples which would be classified as fallacies under the 4i system, but which the Carey system might fail to include.

Dr Anthony Yeung of the Hong Kong Polytechnic University is the author of the chapter "Some Common Informal Fallacies" in the book Critical Thinking (2005, McGrawHill). In the beginning of the chapter, he defines fallacies as mistakes in inferences that are not obvious. Would you agree with this definition? Why or why not?