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How to Use Deductive Reasoning
What is Deductive Reasoning?
Understanding how deductive reasoning works can be most easily achieved by considering a classic example of this reasoning in action: the work of a criminal detective. When a detective does the work of solving a crime, she makes regular use of both inductive and deductive reasoning. While the actual process is far more integrated and complicated than what I will present here, considering these two forms of reasoning from this basic perspective will create a clear picture of how deductive reasoning works in a general sense.
Understanding Inductive Reasoning
On first arriving at the scene of a crime the detective’s first priority, along with that of the rest of the police force, is to gather evidence. Everyone sets to work collecting, organizing and securing as many details as possible for later study. As the details come together the detective begins to use inductive reasoning to develop theories for how the crime may have taken place.
This is the basic structure of inductive reasoning: the process of analyzing facts and evidence in an attempt to articulate what they suggest might be true. Anytime you take a collection of details and examine them in an attempt to create generalizations or hypothesis about what they infer you are using inductive reasoning. The advantage of this approach comes in its ability to help you develop a list of possibilities for things that might be true given what you currently know. In just this way, the detective uses his evidence to develop competing theories for a given crime.
…analyzing a proposition accepted as true in order to determine what else must also be true as a direct result
Understanding Deductive Reasoning
Once these theories are in place, the process of deductive reasoning begins. In deductive reasoning, you begin with a proposition that you accept as true. If that proposition is in fact true, then it will suggest many other things that must also be true.
Thus, when the detective begins to examine a particular theory for a crime using deductive reasoning, it provides her with a focused direction for further questions and gathering of evidence. As the process proceeds, if something is found that doesn’t fit, it disproves the original proposition, allowing the detective to rule that theory out and move on to others.
Eventually, as these two processes weave together, the detective eventually resolves the crime by using inductive reasoning to come up with the theories and using deductive reasoning to prove their validity.
Thus, inductive reasoning is about figuring out what might be true where deductive reasoning is about figuring out what must be true. This definition is a bit oversimplified, but it gets at the basic idea and provides a reasonable foundation for understanding the rest of this article.
Deductive Reasoning as a Testing Strategy
In January of 2012 I received the results of a comprehensive multiple-choice test on reading and writing skills for the students at the school where I teach. The first semester had gone well, I’d made some solid research-based changes to the way I was teaching and I was expecting good results.
The scores were dismal.
After several days of frustration, self-incrimination and general whining, I took a serious look at the results and searched for an explanation. Days of number crunching led me to one basic conclusion: my students were not showing what they know. While I wouldn’t expect their scores to be perfect, they should have been significantly higher.
In short order, I discovered that their central problem was not one of knowledge or skill base, but one of not understanding how to take a test. My students were failing to show what they know. After teaching this lesson on deductive reasoning, their scores literally doubled, finally reaching a level that I felt was a genuine reflection of what they actually knew and understood about reading and writing.
Here is the lesson:
How Not to Take a Test
The Worst Way to Take a Test
Short of wadding it up and throwing it away, the very worst way to take a test is to simply guess randomly. Statistics suggest that you can earn about a 25% on a test with this approach. In my experience, it’s much worse. Students who take this approach can’t seem to resist the temptation to use the answer bubbles to draw clever pictures of hippos, which, while quite creative, does not bode well for one’s test score.
The Next Worst Way to Take a Test
The next best approach is not much better. Many students I have will scan the given reading or problem, then run down the list of answers looking for the one that could be correct. Once they find one they mark it, rarely taking the time to even read the other answers, let alone deeply consider them.
Yet Another Bad Way to Take a Test
Most of the students, however, do not fall into either of these two categories. Instead, they take this approach. They begin by reading the questions. Then, if applicable, they will read the given text for the questions. They will then read the answers and decide which one is most likely to be true. Whichever answer they decide upon is the one they pick, and then they move on to the next question.
If you recall from our initial discussion above about deductive and inductive reasoning, this approach is largely inductive. That is, the student is looking at the details and thinking about what is most likely to be true. Also recall, however, that inductive reasoning is designed to help people discover what might be true, not what is actually true.
Thus, getting the wrong answer is pretty common, especially with more difficult questions in which there may be several “correct” answers, but one is actually more correct when examined closely. Quite simply, this is a very bad way to take a test.
Text Annotation Quick Reference
Symbol to Use
What to Use it For
Mark Questions or Confusing Parts
Mark Ideas that Strike You
Underline Key Words / Phrases
Important Ideas & Phrases
Write Out Connecting Words
Note Personal Connections
How to Take a Test
Step 1: Annotate the Text
Read the problem carefully, carefully read the problem and read the problem with great care. Am I repeating myself? Yes. I think I am, so let’s say it one more time: read the problem carefully!
If you intend to use deductive reasoning successfully as a testing strategy, you must begin by being very clear about what the problem is asking. If it’s a math problem, analyze it. If it’s a text to read, read it slowly and deeply. In both cases, annotate the text if at all possible (this means, write notes directly on the given problem or text).
To give you the idea of what this should look like, watch this short video in which I model the process:
Annotation Demonstration Video
Step 2: Use Inductive Reasoning to Survey the Answers
With a solid sense of the question now in place, read through the answers to determine which of them might be true and which are definitely false. If possible, put a slash mark through ones that are flat wrong and put a question mark beside ones that might be correct.
Notice that I did not say “are” correct. At this stage, you cannot know for certain which answer is actually correct, so it’s important to keep in mind that you are looking for possible answers not final answers.
Step 3: Use Deductive Reasoning to Determine the Right Answer
Now that you have narrowed down the list of possible answers, it’s time to determine which of them is actually correct. Here is where deductive reasoning’s power to reveal what is actually true versus what may be true comes in very handy.
To use this approach correctly, you have to change the way you think about the answers. That is, instead of searching for the one you think is the most correct, you take each of the answers you have identified as possibilities and deal with them each individually.
So, if I think that “B” is a possible answer, then I begin my thinking with the assumption that “B” is, in fact, correct. With this in mind, I go back into the details of the question and its text to “prove” it. Essentially, I go searching for the evidence that makes this answer true. If I can find it, then it can stay. If I can’t find it, or something else in the evidence contradicts it, then it fails and I cross it out. Either way, I then move on to the next possible answer and follow the same procedure.
By taking this approach, you will know with certainty why the correct answer you chose is, in fact, correct. As an added bonus for those really tough questions, you will find that you are far better prepared to measure two good answers against one another and decide on the best option.
A Final Note
Let's be honest, taking a test this way takes a lot more time, which is something that most students really hate to give up. However, the truth of the matter is this: I guarantee that taking tests this way will improve your test scores because it allows you to actually use all that you know. And every reluctant student I basically force to take tests this way is smiling when they walk away with their score because it’s higher than what they’re used to.
School is hard work.
So is life.
It will turn out much better that way!
How Deductive Reasoning can Reduce Test Anxiety
In my experience, test anxiety generally has one of two sources: lack of preparation or fear of the testing process itself. For those who suffer from the first problem, my suggestion is that you study. I wish I could offer a better solution, but there isn’t one. If you’re not ready for your tests, then get ready—period. As I said before, school is hard work, and so is life—deal.
For those who suffer from fear of the process itself, the advantage of deductive reasoning is that it takes much of the mystery and surprise out of testing by giving you a very specific process to follow for each question. Simply work through the process and you are guaranteed to get the chance to fully show what you know.