Decision Making Tool: Occam's Razor

Learn and practice using Occam's Razor with exercises.

What is Occam’s Razor?

The concept in one sentence:

The simplest explanation is more likely to be true.

The concept in one quote:

The explanation requiring the fewest assumptions is most likely to be correct.

William of Ockham

The concept in one image:

How I use it?

My mind tends to jump to worst-case scenarios. By recalling Occam’s Razor, I can stop assuming the worst and start being rational.


Examples

Example 1

Your mum went on a day trip with friends. You texted her but never got a reply.

Which is more likely?

  1. Her phone’s battery died.

  2. She broke her phone while running to catch a train.

Answer:

The correct answer is option 1.

Option 1 requires fewer assumptions—the battery died.

Whereas option 2 assumes that she had to catch a train, ran, and broke her phone.

Example 2

You went shopping and left your dog at home. When you got back home, the house was messy.

Which is more likely?

  1. A burglar broke in and searched your home for passports.

  2. Your dog made a mess.

Answer:

The correct answer is option 2.

Option 2 requires fewer assumptions—the dog made a mess.

Whereas option 1 assumes that a burglar broke in and that the burglar searched your home for passports.

Example 3

You arrived at work feeling irritated.

Which is more likely?

  1. You didn’t have enough sleep.

  2. You drove to work and had an argument with another driver.

Answer:

The correct answer is option 1.

Option 1 requires fewer assumptions—not enough sleep.

Whereas option 2 assumes that you drove to work and that you had an argument with another driver.


Exercises

Exercise 1

Linda is 31 years old, single, outspoken, and very bright. She majored in finance. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations.

Which is more likely?

  1. Linda is a financial analyst.

  2. Linda is a financial analyst and is active in the feminist movement.

Exercise 2

Which is more likely?

  1. I got a bad score on my test because the teacher doesn’t like me.

  2. I got a bad score on my test because I didn’t study.

Exercise 3

Joe likes to cook and run marathons.

Which is more likely?

  1. Joe is a chef.

  2. Joe is a chef and walks to work.

Exercise 4

Which is more likely?

  1. Humans built pyramids.

  2. Aliens built the pyramids.

Exercise 5

They are 3 candidates running for president, Anna, Bob, and Chris. The latest polls put their chances at:

  • Anna - 45%

  • Bob - 40%

  • Chris - 15%

Which is more likely?

  1. Anna or Bob wins.

  2. Anna, Bob, or Chris wins.

Exercise 6

I wrote over 100 books.

Which is more likely?

  1. I am an author.

  2. I am an author and I sold books.

Exercise 7

Despite your efforts, you had 3 bad relationships in a row.

Which is more likely?

  1. Relationships are bad.

  2. You chose the wrong person to be with, 3 times in a row.

Exercise 8

Several murders are committed in your town. Every victim is found with a red cross on their nose.

Which is more likely?

  1. Same killer.

  2. Different killers.

Exercise 9

A friend of a friend arrived late to your birthday party.

Which is more likely?

  1. They don’t like you

  2. They were delayed.

Exercise 10

You live with your partner and kid. Cookies are missing from the cookie jar.

Which is more likely?

  1. Your kid ate them.

  2. Your partner or kid ate them.

Exercise 11

Your card was declined. You go check your bank balance and see $0.

Which is more likely?

  1. Someone got access to your account and took all your money.

  2. You spent all your money.

Exercise 12

You went camping for a month. Upon your return, you find the roads empty.

Which is more likely?

  1. Lockdown

  2. Zombie apocalypse

Exercise 13

Which is more likely?

  1. We went to the moon.

  2. We never went to the moon and there’s a global conspiracy to hide the truth.

Exercise 14

You are a beginner at tennis. Next week, you have a match against Roger Federer.

Order the following outcomes from most to least likely:

  1. Roger Federer will win the match.

  2. Roger Federer will lose the first set.

  3. Roger Federer will lose the first set but win the match.

  4. Roger Federer will win the first set but lose the match.

Exercise 15:

You are at home and can’t find your phone. You were playing on it 5 hours ago in the kitchen.

Which is more likely?

  1. You misplaced it.

  2. Someone stole it.


Answers

Exercise 1:

The correct answer is option 1.

Option 1 requires fewer assumptions—she’s a bank teller.

Whereas option 2 assumes that Linda is a bank teller and that she is active in the feminist movement.

Exercise 2:

The correct answer is option 2.

Option 2 requires fewer assumptions—I didn’t study.

Whereas option 1 assumes that the teacher doesn’t like me and that it would make them give me a bad score.

Exercise 3:

The correct answer is option 1.

Option 1 requires fewer assumptions—he’s a chef.

Whereas option 2 assumes that Joe is a chef and that he walks to work.

Exercise 4:

The correct answer is option 1.

Option 1 requires fewer assumptions—humans built the pyramids.

Whereas option 2 assumes that alien exists and that they built the pyramids.

Exercise 5:

The correct answer is option 2.

Option 2 requires fewer assumptions—one of them will win.

Whereas option 1 assumes that either Anna or Bob will win and that Chris will lose.

Exercise 6:

The correct answer is option 1.

Option 1 requires fewer assumptions—I am an author.

Whereas option 2 assumes that I am an author and that I sold books.

Exercise 7:

The correct answer is option 2.

Option 2 requires fewer assumptions—you chose the wrong person, 3 times in a row.

Whereas option 1 assumes that every relationship is bad.

Exercise 8:

The correct answer is option 1.

Option 1 requires fewer assumptions—the same killer.

Whereas option 2 assumes that different killers committed the murders and that each of them left a red cross on the victim’s nose.

Exercise 9:

The correct answer is option 2.

Option 2 requires fewer assumptions—they were delayed.

Whereas option 1 assumes that they don’t like you and that they arrived late to spite you.

Exercise 10:

The correct answer is option 2.

Option 2 requires fewer assumptions—one of them ate the cookies.

Whereas option 1 assumes that your kid ate them and that your partner didn’t.

Exercise 11:

The correct answer is option 2.

Option 2 requires fewer assumptions—you spent all your money.

Whereas option 1 assumes that someone got access to your bank account and was able to take all your money.

Exercise 12:

The correct answer is option 1.

Option 1 requires fewer assumptions—lockdown.

Whereas option 2 assumes that zombies exist, and they are the reason why the roads are empty.

But just in case you need it: Zombie Preparedness - Centers for Disease Control and Prevention (CDC)

Exercise 13:

The correct answer is option 1.

Option 1 requires fewer assumptions—we went to the moon.

Whereas option 2 assumes that all the evidence concerning NASA’s moon landing is fake and that a large number of people are all conspiring to hide the truth.

Exercise 14:

The correct order is 1, 2, 3, 4.

Outcome 2 (Roger Federer will lose the first set) and outcome 3 (Roger Federer will lose the first set but win the match) are in the right order.

Outcome 2 has one assumption, whereas outcome 3 has 2 assumptions.

Therefore outcome 2 is more likely than outcome 3.

Exercise 15:

The correct answer is option 1.

Option 1 requires fewer assumptions—you misplaced it.

Whereas option 2 assumes that someone came and stole it.


Bonus

Sheldon gives an example of Occam’s Razor.


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