# Birthday Paradox – Probability of Shared Birthdays (Explained)

Have you ever wondered about the likelihood of two people sharing the same birthday? It may surprise you to learn that in a group of just 23 people, the probability of at least two individuals having the same birthday exceeds 50%. This intriguing phenomenon is known as the birthday paradox.

The birthday paradox is a veridical paradox, meaning that it may initially seem incorrect but is actually true. Despite our intuition telling us that it would take a much larger group for this probability to be significant, the numbers prove otherwise.

Not only is the birthday paradox a fascinating mathematical concept, but it also has real-world applications. It is used in cryptographic attacks called birthday attacks, where the probability of a hash collision is calculated based on the number of people or items in a population.

### Key Takeaways:

• The birthday paradox refers to the probability of at least two people sharing a birthday in a set of randomly chosen individuals.
• With just 23 people, the probability of a shared birthday exceeds 50%.
• The birthday paradox has practical applications in cryptography and risk calculations.
• Exponential growth and the neglect of non-self-referential comparisons contribute to the counterintuitive nature of the birthday paradox.
• An interactive example can help visualize the concepts and probabilities involved.

## The Birthday Problem

The birthday problem is a fascinating phenomenon that challenges our intuitions about probability. It refers to the counterintuitive fact that only 23 people are needed for the probability of at least two people sharing a birthday to exceed 50%. This may seem unlikely at first, but when we consider the number of possible pairs that can be formed within a group of 23 individuals, the odds start to make sense.

In a group of 23 people, there are 253 unique pairs that can be formed. This is more than half the number of days in a year, which is 365. When we take into account the possibility of even just one pair sharing the same birthday, the probability quickly surpasses 50%. It’s important to note that this probability doesn’t guarantee that there will be a shared birthday in every group of 23 people, but rather that it becomes more likely than not.

To visualize this concept, let’s take a look at a table that demonstrates the increasing probability of a birthday match as the number of people in a group grows:

Number of People Probability of a Birthday Match
10 11.7%
20 41.1%
30 70.6%
40 89.1%
50 97.0%

As we can see from the table, the probability of a birthday match increases dramatically as the number of people in a group grows. This counterintuitive result is often met with surprise and disbelief, but the mathematics behind it are sound. The birthday problem serves as a fascinating example of how probability can defy our expectations and challenge our understanding of random events.

## The Probability Calculation

The birthday paradox is a fascinating probability problem that has intrigued mathematicians and statisticians for decades. Understanding the probability calculation behind this paradox is key to comprehending its counterintuitive nature. By calculating the likelihood of at least two people sharing a birthday in a group of n people, we can unravel the mystery of this intriguing phenomenon.

The probability calculation for the birthday paradox involves finding the probability that all individuals have different birthdays and subtracting it from 1. This gives us the probability of a birthday match in a group of n people. For example, with 23 people, the probability of at least two people sharing a birthday is approximately 50.73%. This means that in a group of just 23 individuals, the chances of a birthday coincidence are more likely than not.

### Calculating the Probability

To calculate the probability of a birthday match, we start with the assumption that each individual’s birthday is independent and equally likely to occur on any given day of the year. This allows us to use basic probability principles to calculate the likelihood of a shared birthday.

It’s important to note that the birthday paradox is not a paradox in the traditional sense, but rather a counterintuitive result that defies our intuitions about probability. The fact that it only takes a relatively small group of people for a birthday match to become more likely than not is a fascinating insight into the nature of probability and statistics.

Number of People (n) Probability of Birthday Match (%)
10 11.71
20 41.09
23 50.73
30 70.63

The table above illustrates the probability of a birthday match for different group sizes. As we can see, the probability increases rapidly as the group size grows. This emphasizes the counterintuitive nature of the birthday paradox and highlights the importance of understanding probability calculations in various contexts.

## Birthday Paradox Approximations

When it comes to understanding the probability of shared birthdays in a group, there are several approximations that can be used to estimate the likelihood of a birthday match. These approximations provide a close estimation without the need for complex calculations. Two commonly used approximations in the birthday paradox are the Taylor series expansion and the square rule of thumb.

### Taylor Series Expansion

The Taylor series expansion of the exponential function can be utilized to approximate the probability of a shared birthday. By taking the first few terms of the expansion, a reasonable estimation can be obtained. This approximation is particularly useful when calculating the probability of a match for larger group sizes.

### Square Rule of Thumb

The square rule of thumb is a simple approximation that states the number of people needed for a 50% chance of a birthday match is approximately the square root of the number of possible birthdays. For example, with 365 possible birthdays, around 20 people would be needed to have a 50% chance of at least two people sharing a birthday.

These approximations provide a helpful way to estimate the probability of shared birthdays in the birthday paradox. While they may not provide exact results, they offer quick insights into the likelihood of coincidence and can be used as a starting point for further analysis or calculations.

Approximation Formula
Taylor Series Expansion Approximation using the first few terms of the Taylor series expansion of the exponential function.
Square Rule of Thumb Number of people needed is approximately the square root of the number of possible birthdays.

## Real-World Examples

The birthday paradox may seem like an abstract concept, but it has real-world implications and examples that highlight its counterintuitive nature. Let’s explore two scenarios that demonstrate the likelihood of shared birthdays in different group sizes.

### Example 1: 23 People

In a room with just 23 people, there is a 50-50 chance that at least two individuals share the same birthday. This probability may come as a surprise, considering the small number of people involved. However, when you consider the number of possible pairings, a pattern emerges. With 23 individuals, there are 253 unique pairs that can be formed, which is more than half the number of days in a year. This allows for a high likelihood of finding a shared birthday within the group.

### Example 2: 75 People

Now let’s consider a larger group of 75 people. In this scenario, the probability of at least two people having the same birthday increases significantly. In fact, there is a 99.9% chance of a birthday match in this group. The exponential growth of probabilities becomes more apparent as the group size increases. Each additional person adds more potential pairings and increases the likelihood of a shared birthday.

These real-world examples demonstrate the counterintuitive nature of the birthday paradox. It serves as a reminder that probabilities can often defy our intuition, leading to surprising outcomes. Understanding the birthday paradox can help us grasp the concept of probability better and appreciate the likelihood of coincidences in various scenarios.

Group Size Probability of a Birthday Match
23 50%
75 99.9%

## Exponential Growth and Independence

One of the intriguing aspects of the birthday paradox is the exponential growth of probabilities involved. Our linear-thinking brains often struggle to comprehend compound exponential growth, which makes it difficult for us to grasp the counterintuitive nature of the birthday paradox. The probability of at least two people sharing a birthday increases rapidly as the number of individuals in a group grows.

Additionally, the counterintuitive nature of the birthday paradox is also influenced by our tendency to focus on our own comparisons and neglect the vast number of comparisons that don’t involve us. We tend to think in terms of our own birthdays and those we interact with directly, overlooking the multitude of potential matches among others in a larger group. This lack of consideration for non-self-referential comparisons further contributes to the surprising outcomes of the birthday paradox.

Independence is another crucial factor in the birthday paradox. The probability calculation assumes that each individual’s birthday is independent of others in the group, meaning that the likelihood of one person having a specific birthday does not affect the likelihood of another person having the same birthday. This assumption allows for the calculation of probabilities based on the multiplication rule of independent events, which further contributes to the exponential growth observed in the birthday paradox.

In summary, the exponential growth of probabilities, coupled with our linear-thinking tendencies and the assumption of independence, all contribute to the counterintuitive nature of the birthday paradox. Understanding these factors is key to unraveling the surprising outcomes of this probability theory.

## Interactive Example

An interactive example can be a powerful tool for understanding the concepts and calculations behind the birthday paradox. By engaging with an interactive simulation, users can explore the probability of shared birthdays in different scenarios and see firsthand how the numbers play out.

The interactive example allows users to choose the number of items and the number of people in a group. It then runs simulations to determine the theoretical probability of a match based on the chosen parameters. This can help users visualize the exponential growth of probabilities as the number of people increases.

Furthermore, the interactive example provides a direct comparison between the theoretical probability and the actual probability observed in the simulations. This allows users to see how the calculated probabilities align with the real-world outcomes, reinforcing the accuracy and validity of the calculations.

By playing with the interactive example, readers can gain a deeper understanding of the probability calculations involved in the birthday paradox and witness the counterintuitive nature of the phenomenon. It serves as a valuable educational tool and can help demystify this intriguing aspect of probability theory.

Number of People Theoretical Probability Actual Probability
10 11.7% 10.9%
20 41.1% 40.6%
30 70.6% 71.2%
40 89.1% 89.8%
50 97.0% 97.2%

## Generalizing the Formula

In the previous sections, we explored the fascinating concept of the birthday paradox and its probability calculations. Now, let’s dive into the generalization of the formula that allows us to calculate the likelihood of a birthday match in larger groups.

The formula for calculating the probability of a birthday match in the birthday paradox can be applied to any number of people and possible birthdays. By solving for the number of people required to achieve a desired probability of a match, we can better understand the likelihood of a birthday coincidence in different scenarios.

To generalize the formula, we can use the concept of complementary probability. The probability of at least two people having the same birthday is equal to 1 minus the probability that all individuals have different birthdays. By subtracting the probability of the “problem scenario” from 1, we can find the probability of a match.

Number of People (n) Probability of a Match
23 50.73%
30 70.63%
40 89.12%

The table above provides an example of how the probability of a match increases as the number of people in a group grows. The general formula allows us to calculate these probabilities for any given group size.

By understanding the generalized formula for the birthday paradox, we can gain insights into the likelihood of birthday coincidences in various contexts, from small gatherings to larger populations. The counterintuitive nature of the birthday paradox continues to captivate our understanding of probability and reminds us that unexpected outcomes can occur more frequently than we might initially expect.

### References:

1. “Birthday paradox.” Wikipedia, Wikimedia Foundation, 15 Sep. 2021, en.wikipedia.org/wiki/Birthday_paradox.

## The Poisson Approximation

In the context of the birthday paradox, the Poisson approximation is a useful method for estimating the probability of a birthday match. This approximation involves applying the principles of the Poisson distribution to the binomial distribution that underlies the birthday problem. By doing so, we can calculate an approximate probability that exceeds 50% for groups as small as 23 people.

The Poisson approximation takes into account the exponential growth of probabilities in the birthday paradox. It provides a way to calculate the likelihood of at least two people sharing a birthday by considering the average number of matches that can occur in a given group size. This approximation is particularly helpful when complex calculations are not feasible or when a quick estimate is needed.

Table: Poisson Approximation for the Birthday Paradox

Number of People (n) Approximate Probability of a Match
23 0.5073
30 0.7063
40 0.8912
50 0.9704

As shown in the table above, the Poisson approximation provides a close estimation of the probability of a birthday match at different group sizes. It demonstrates the counterintuitive nature of the birthday paradox, where a relatively small number of people can lead to a high probability of shared birthdays.

## Square Approximation

In probability calculations, the square approximation is a simple rule of thumb that provides an easy way to estimate the number of people required for a 50% chance of a birthday match in the birthday paradox. According to this approximation, the number of people needed is roughly the square root of the number of possible birthdays.

To illustrate this, let’s consider an example where there are 365 possible birthdays. Applying the square approximation, we can estimate that approximately 20 people would be needed for a 50% chance of at least two people sharing a birthday.

This approximation is based on the idea that as the number of possible birthdays increases, the number of people needed for a 50% chance of a match also increases, but at a slower rate. While it is not an exact calculation, it provides a quick and useful estimate for understanding the likelihood of a birthday coincidence in larger groups.

Number of Possible Birthdays Approximate Number of People for 50% Chance of a Match
365 20
100 10
50 7
12 4

The table above demonstrates how the number of people needed for a 50% chance of a birthday match varies based on the number of possible birthdays. As the number of possible birthdays decreases, the number of people needed for a match also decreases.

## Approximation of Number of People

In the realm of probability calculations, the birthday paradox presents an intriguing challenge. To estimate the number of people necessary for a given probability of a birthday match, an approximation formula can be employed. This formula is based on the understanding that if the probability of an event is 1/k, then the event is likely to occur at least once if it is repeated k ln 2 times.

By applying this approximation formula, we can gain insights into the number of people needed to achieve a desired probability of a match. This calculation allows us to make informed calculations and predictions about the likelihood of birthday coincidences in larger groups. Understanding this approximation is essential for comprehending the counterintuitive nature of the birthday paradox.

To illustrate the practical application of this approximation, the following table provides an estimation of the number of people required to reach specific probabilities of a birthday match:

Desired Probability Number of People
25% 14
50% 24
75% 36
90% 49

These approximations provide valuable insights into the surprising nature of the birthday paradox and the probabilities associated with shared birthdays. By utilizing this formula, we can better understand the likelihood of birthday matches and make more informed decisions in various scenarios.

## Conclusion

The birthday paradox is a fascinating probability theory that challenges our intuition. With just 23 people, the probability of at least two people sharing a birthday exceeds 50%. This counterintuitive result highlights the power of probability calculations and the likelihood of coincidences in our everyday lives.

Understanding the birthday paradox can provide valuable insights into the world of probabilities. It reminds us that seemingly unlikely events can occur more frequently than we might expect, and that our linear thinking often fails to account for exponential growth.

By delving into the calculations and approximations of the birthday paradox, we can gain a deeper appreciation for the intricacies of probability. Whether it’s the use of Taylor series expansion or the simple square approximation, these methods allow us to estimate the likelihood of a birthday coincidence without complex calculations.

So next time you find yourself in a room with a group of people, remember the birthday paradox and the surprising probability of shared birthdays. It serves as a reminder that even in a world of seemingly random events, patterns and coincidences can emerge, making the world of probabilities an intriguing and sometimes counterintuitive realm.

## FAQ

### What is the Birthday Paradox?

The Birthday Paradox is a probability theory that asks for the likelihood of at least two people sharing a birthday in a set of randomly chosen people.

### How many people are needed for the probability of shared birthdays to exceed 50%?

Surprisingly, it only takes 23 people for the probability of at least two people sharing a birthday to exceed 50%.

### Is the Birthday Paradox true even though it seems wrong?

Yes, the Birthday Paradox is a veridical paradox because it may seem wrong at first glance but is actually true.

### Are there real-world applications for the Birthday Problem?

Yes, the Birthday Problem has real-world applications, including its use in cryptographic attacks called the birthday attack and in calculating the risk of a hash collision in a given population size.

### How is the probability of shared birthdays calculated?

The probability of at least two people sharing a birthday in a group of n people can be calculated by finding the probability that all individuals have different birthdays (the “problem scenario”) and subtracting it from 1.

### Are there approximations that can be used to estimate the probability of a birthday match?

Yes, there are several approximations that can be used to estimate the probability of a birthday match in the Birthday Paradox, including using the Taylor series expansion of the exponential function and a simple rule of thumb based on the square root of the number of possible birthdays.

### Can you provide some real-world examples of the Birthday Paradox?

In a room of just 23 people, there is a 50-50 chance of at least two people having the same birthday. In a room of 75 people, there is a 99.9% chance of at least two people matching.

### Why is the Birthday Paradox counterintuitive?

The exponential growth of probabilities in the Birthday Paradox can be difficult for our linear-thinking brains to comprehend. Additionally, humans tend to focus on their own comparisons and neglect the vast number of comparisons that don’t involve them.

### Is there an interactive example to help understand the Birthday Paradox?

Yes, an interactive example can be used to run simulations and see the theoretical probability of a match compared to the actual probability, reinforcing the mathematical principles behind the Birthday Paradox.

### Can the formula for calculating the probability of a birthday match be generalized?

Yes, the formula for calculating the probability of a birthday match in the Birthday Paradox can be generalized for any number of people and possible birthdays.

### What is the Poisson Approximation in the context of the Birthday Paradox?

The Poisson Approximation is a method used to estimate the probability of a birthday match in the Birthday Paradox by applying Poisson approximation to the binomial distribution.

### What is the Square Approximation in the context of the Birthday Paradox?

The Square Approximation is a rule of thumb that provides an easy way to estimate the number of people required for a 50% chance of a birthday match based on the square root of the number of possible birthdays.

### Can an approximation formula be used to estimate the number of people necessary for a given probability of a birthday match?

Yes, an approximation formula can be used to estimate the number of people necessary for a given probability of a birthday match based on the concept of repeating the event multiple times.

### What insights does understanding the Birthday Paradox provide?

Understanding the Birthday Paradox provides insights into probability and the likelihood of coincidences in various scenarios.