Inductive and Deductive Reasoning

Many people distinguish between two basic kinds of argument: inductive and deductive. Induction is usually described as moving from the specific to the general, while deduction begins with the general and ends with the specific; arguments based on experience or observation are best expressed inductively, while arguments based on laws, rules, or other widely accepted principles are best expressed deductively. Consider the following example:
Adham: I've noticed previously that every time I kick a ball up, it comes back down, so I guess this next time when I kick it up, it will come back down, too.

Rizik: That's Newton's Law. Everything that goes up must come down. And so, if you kick the ball up, it must come down.

Adham is using inductive reasoning, arguing from observation, while Rizik is using deductive reasoning, arguing from the law of gravity. Rizik's argument is clearly from the general (the law of gravity) to the specific (this kick); Adham's argument may be less obviously from the specific (each individual instance in which he has observed balls being kicked up and coming back down) to the general (the prediction that a similar event will result in a similar outcome in the future) because he has stated it in terms only of the next similar event--the next time he kicks the ball.

As you can see, the difference between inductive and deducative reasoning is mostly in the way the arguments are expressed. Any inductive argument can also be expressed deductively, and any deductive argument can also be expressed inductively.

Even so, it is important to recognize whether the form of an argument is inductive or deductive, because each requires different sorts of support. Adham's inductive argument, above, is supported by his previous observations, while Rizik's deductive argument is supported by his reference to the law of gravity. Thus, Adham could provide additional support by detailing those observations, without any recourse to books or theories of physics, while Rizik could provide additional support by discussing Newton's law, even if Rizik himself had never seen a ball kicked.

The appropriate selection of an inductive or deductive format for a specific first steps toward sound argumentation.


Exercises for Induction and Deduction

1. Which of the following claims would be best expressed by inductive reasoning?

 Your first quiz grade usually indicates how you will do in the course.

  The final exam accounts for 30% of the course grade.

  Late papers will not be accepted.

  Gravity's Rainbow is required reading in your course.

2. Which of the following claims would be best expressed by deductive reasoning?

 California's population growth rate slowed last year.

 California residents appreciate their good weather.

 California residents are residents of the United States.

 More cars are registered in California than in any other state.

3. Which of the following arguments would lead to a deductive conclusion?

 There was a mild winter this year, and previously whenever there's been a mild winter the cherry crop suffers.

 The cherry crop needs at least a week of freezing temperatures for best results, and this winter the temperature stayed several degress above freezing.

 Primo noticed that whenever the skiing was good in the winter, the cherry crop was profitable, and this year the skiing was good.

 Not since 1972 have I seen a good cherry crop after a mild winter, and this winter has been mild.

1. Which of the following claims would be best expressed by inductive reasoning? You answered:

 Your first quiz grade usually indicates how you will do in the course.

Correct!

It is much more likely that this claim stems from personal observation than from adherence to some general principle, and so it would be best expressed by an inductive argument.

2. Which of the following claims would be best expressed by deductive reasoning? You answered:

 California residents are residents of the United States.

Correct!

Since California residents are residents of the U.S. by definition, and definition is one sort of general claim on which a deductive argument can be based, this claim would be best supported by a deductive argument.

3. Which of the following arguments would lead to a deductive conclusion? You answered:

 The cherry crop needs at least a week of freezing temperatures for best results, and this winter the temperature stayed several degress above freezing.

Correct!

Though the premise about this year is based on observation, the general or "major" premise, "The cherry crop needs at least a week of freezing temperatures," seems to be an assertion of established principle, making this a deductive argument with the conclusion, "The results won't be best this year."

1. Which of the following claims would be best expressed by inductive reasoning? You answered:

  The final exam accounts for 30% of the course grade.

On which premise is it morely likely that someone would base that claim: "In my experience, final exams have counted for 30% of the course grade," or "The instructor has stipulated that the final exam counts for 30% of the course grade"? Since, unlike in the case of gravity, experience is not a good predictor of the way grades are determined, it is unlikely that this claim would be argued inductively.

1. Which of the following claims would be best expressed by inductive reasoning?

 Gravity's Rainbow is required reading in your course.

On which premise is it morely likely that someone would base that claim: "In my experience, Gravity's Rainbow has been required reading in such courses," or "The instructor has stipulated that Gravity's Rainbow is required reading"? Since, unlike in the case of gravity, experience is probably not a good predictor of what is assigned as reading in a course, it is unlikely that this claim would be argued inductively.

1. Which of the following claims would be best expressed by inductive reasoning?

  Late papers will not be accepted.

On which premise is it morely likely that someone would base that claim: "In my experience, late papers have not been accepted," or "The instructor has stipulated that late papers will not be accepted"? Since, unlike in the case of gravity, experience is not necessarily a good predictor of course requirements, it is unlikely that this claim would be argued inductively.

2. Which of the following claims would be best expressed by deductive reasoning? You answered:

 California's population growth rate slowed last year.

Since this claim would probably be based on a study of California's population growth rate, and a study is just a formal kind of observation, the argument made to support it would most likely be inductive. Of course, we could make a general principle such as "The population growth rate slowed in all 50 states last year" and then deduce that it therefore slowed in California, which is one of the 50 states. But such a deduction would be less likely, in part because the rate of growth is information best established by survey and observation.

2. Which of the following claims would be best expressed by deductive reasoning?

 California residents appreciate their good weather.

Since this claim would probably be based on a study of California residents' attitudes, and a study is just a formal kind of observation, the argument made to support it would most likely be inductive. Of course, we could make a general principle such as "All U.S. residents appreciate their good weather," and then deduce that Californians, who are by definition U.S. residents, therefore must appreciate their good weather, too. But such a deduction would be less likely, in part because attitudes, especially about such things as the weather, is information best established by survey and observation.

2. Which of the following claims would be best expressed by deductive reasoning? You answered:

 More cars are registered in California than in any other state.

Since this claim would probably be based in some way on a study of the number of cars registeed in various states, and a study is just a formal kind of observation, the argument made to support it would most likely be inductive. Of course, we could make a general principle such as "The state with the largest population has the greatest number of registered cars," and then deduce that California, the most populous state, therefore has the most cars. But such a deduction would be less likely, in part because both the size of the population and the number of cars are the sort of information established by survey and observation.

3. Which of the following arguments would lead to a deductive conclusion? You answered:

 There was a mild winter this year, and previously whenever there's been a mild winter the cherry crop suffers.

This is very close to a deductive argument, but "previously" suggests that the general premise here is based on observation and not an established principle. The equivalent claim for a deductive argument would be, "Whenever there is a mild winter, the cherry crop suffers."

3. Which of the following arguments would lead to a deductive conclusion? You answered:

 Primo noticed that whenever the skiing was good in the winter, the cherry crop was profitable, and this year the skiing was good.

The more general of the two claims here, "whenever the skiing was good in the winter, the cherry crop was profitable," is clearly based on individual observation--in this case, Primo's. Such observations are usually the basis for inductive arguments. A deductive argument along these lines might assert more simply, "Whenever the skiing is good in the winter, the cherry crop is profitable." If the second premise is, as here, "this year the skiing was good," the conclusion would be "the cherry crop will be profitable."

3. Which of the following arguments would lead to a deductive conclusion? You answered:

 Not since 1972 have I seen a good cherry crop after a mild winter, and this winter has been mild.

The basis for the more general of the two claims here, "Not since 1972 have I seen a good cherry crop after a mild winter," is clearly personal observation, and so would be best utilized in an inductive argument. The premise for a similar deductive argument might be, "There are no good cherry crops after a mild winter."

Congratulations!

You have finished the section on Introduction to Induction and Deduction. To continue with the next section of "Mission: Critical," click on "Induction," above.