What is Apriori Algorithm With Example? Simplest Explanation!

In this article, I will explain What is the Apriori Algorithm With Example?. I tried to explain the Apriori Algorithm in the easiest way so that you can understand the whole concepts of the apriori algorithm easily.

So, without further ado, let’s get started-

What is Apriori Algorithm With Example?

Apriori algorithm is the algorithm that is used to find out the association rules between objects. That means how two objects are associated and related to each other. In simple words, the apriori algorithm is an association rule learning that analyzes that “People who bought item X also bought item Y.

The objective of the apriori algorithm is to generate the association rule between objects. And the association rule tells us how two or three objects are correlated to each other. Apriori Algorithm is also known as frequent pattern mining.

Where Apriori Algorithm is Used?

The most common and popular example of the apriori algorithm is Recommendation System. Let’s understand with the help of the Movie Recommendation example.

Suppose this is the data of users who like some movies-

User IdMovies Liked
111Movie 1, Movie 2, Movie 3, Movie 4
112Movie 1, Movie 2
113Movie 1, Movie 2, Movie 4
114Movie 1, Movie 3
115Movie 2, Movie 4
Table 1

So from this data, we can generate some association rules that the person who likes Movie 1 also likes Movie 2, and people who like Movie 2 are quite likely to also like Movie 4, and so on.

We can generate many rules with the help of this data, some rules are weak and some rules are strong. So for business decisions, only strong rules are used.

And here the question comes in your mind- How to filter strong rules from the weaker ones?


So, let’s understand the whole working of the Apriori Algorithm in the next section with the help of an example-

How Apriori Algorithm Works?

Before I discuss the working of the apriori algorithm, you should remember two main concepts of the Apriori algorithm and that is-

  • Support
  • Confidence

The whole working of the apriori algorithm is based on these terms. So, let’s see What is Support, and confidence, in the Apriori Algorithm with respect to movie recommendation example-

Support= People who watch Movie 1/ Total no. of users

Confidence (M1-> M2)= People who watch Movie 1 & Movie 2/ People who watch Movie 1

Now, you knew about the terms that are used in the Apriori Algorithm. So let’s understand how the apriori algorithm works with the help of an example-

Suppose this is our dataset of any supermarket, where user id and items are listed-

User IdItems
0011 3 4
0022 3 5
0031 2 3 5
0042 5
Table 2

In this data, the user 001 purchased items 1,3, and 4. The user 002 purchased items 2,3, and 5, and so on. So here we have to find the shopping pattern between these items 1,2,3,4, and 5.

Step 1-

So, the first step in the apriori algorithm is to set minimum support and confidence. This will act as a threshold value. By setting minimum support and confidence, you can avoid items that have less support than the threshold value. And here you got an answer to the question- How to filter out strong rules from the weak rules?– by setting minimum support and confidence, you can filter out strong rules from the weak rules.

Suppose I set minimum support as 50% and confidence as 70%.

Step 2-

Now the next step is to calculate the support of each item 1,2,3,4, and 5. So I calculated the support of each item in the following table-

12/4= 50%
23/4= 75%
Table 3

Confused…? How I calculated the support?

What is the Apriori Algorithm With Example?

Don’t Worry!…I’ll explain…So, let’s see how I calculated the support for Item 1-

According to the formula of supportPeople who buy Item 1/ Total no. of users.

So, Item 1 is purchased by 2 people(001 & 003)-> refer Table 2

That’s why it’s 2 and the total no of users is 4, so the support is 2/4=50%. I hope now you understood, similarly you can calculate the support of all other items.

So after calculating the support of all items, we need to check which item has less support than the minimum support threshold. In our example, Item 4 has 25% support that is less than our minimum support.

That’s why I remove item 4 for further steps. Now we have items 1,2,3 and 5.

Step 3-

After calculating the support of each individual item, now we calculate the support of a pair of items. And for that, we need to form pairs.

Table 4

I hope you understood how I formed the pairs. But still, I would like to explain- In step 2, we had 4 items left [1,2,3,5]. So I simply multiplied 1 with all items like {1,2}, {1,3}, {1,5}. Then I multiplied 2 with 3 and 5, so I got {2,3}, and {2,5}. And then 3 is multiplied by 5, so I got {3,5}.

And then for calculating the support of each pair, you need to refer again to table 2. For example, for pair {1,2}, you need to check table 2, how many people bought items 1 & 2 together.

So, according to table 2, only one person bought item 1 & 2 together, that’s why the nominator is 1. And the total no of people is 4, so the denominator is 4. So 1/4=25%. Similarly, I calculated the support of all pairs.

Now it’s time to filter out the pairs who have less support than the minimum support. So pair {1,2} and {1,5} have 25% support. So I eliminate these two pairs for further steps.

Step 4-

Now we have following pairs-{1,3},{2,3}, {2,5}, and {3,5}. And in these pairs, we have item 1,2,3,5. Now it’s time to form triplets with these four(1,2,3,5) items.

Table 5

In this table, I created all possible triplets in the same way as I formed pairs in the previous step. And similarly, I calculated support for all triplets in the same way as I did in the last step. So I think you understood how to form a triplet and calculate support.

Now let’s eliminate the triplets who have support less than the minimum support. And in this case, {1,3,5},{1,2,5}, and {1,2,5} are eliminated. We have only one triplet {2,3,5} who satisfies the minimum support.

Step 5-

Now we need to form an association rule with this triplet-{2,3,5}. As we have only three items, so we can generate rules something like that-

Table 6

In this table, I created rules with three items {2,3,5}. I hope you understood how I created the rules, simply by replacing 2, 3, and 5.

But you might be confused with Support as 2. Right…? So I put support as 2 in all the rules because these rules are generated by the triplet {2,3,5} and this triplet occurs 2 times in Table 2. So the support count of {2,3,5} is 2. That’s why I put support as 2. I hope now you understood.

Now, the next is-

How I calculated the confidence of all rules?
So, let’s understand-

The formula of confidence is= S(AUB)/S(A)

I will explain the calculation for the rule (2^3)->5. Similarly, you can calculate the confidence for all other rules.

So consider (2^3) as A and 5 as B.

According to the formula=S(AUB)/S(A)


= 2/2


Here the support of S(2^3)U5) is 2 because all three items come from triplet {2,3,5} whose support count is 2. And the support of S(2^3) is 2 because the pair {2,3} has support count 2->refer Table 4.

I hope now you understood. Similarly, you can calculate the confidence of other rules. After calculating the confidence of all rules, compare with the threshold value of Confidence. In the beginning, I set the threshold value for confidence as 70%.

So the rules who have less than 70% confidence are eliminated. After eliminating the rules, we have only two rules left that satisfy the threshold value and these rules are-

  • (2^3)->5
  • (3^5)->2

So, these are the two final and strong association rules that are generated by using the Apriori Algorithm. I hope now you understood how the apriori algorithm works.

So, that’s all about Apriori Algorithm. Now it’s time to wrap up!


I tried to write this article in an easy way so that you understand the Apriori Algorithm easily. I hope you understood the whole concept of the Apriori Algorithm. But still, if you have some doubt, feel free to ask me in the comment section.

All the Best!

Happy Learning!

Learn the Basics of Machine Learning Here

Machine Learning A to Z Basics

Thank YOU!

Though of the Day…

Anyone who stops learning is old, whether at twenty or eighty. Anyone who keeps learning stays young.

– Henry Ford
author image

Written By Aqsa Zafar

Founder of MLTUT, Machine Learning Ph.D. scholar at Dayananda Sagar University. Research on social media depression detection. Create tutorials on ML and data science for diverse applications. Passionate about sharing knowledge through website and social media.

2 thoughts on “What is Apriori Algorithm With Example? Simplest Explanation!”

Leave a Comment

Your email address will not be published. Required fields are marked *