Antony’s Personal Opinion

If there are 23 people in a room, what is the probability that two persons share the same birthday?

Can you believe that the probability is more than 50%? Yes… I am not kidding, 50.7% to be exactly. How come??? There are 365 days in a year and there are only 23 people in a room.

I was very surprise too…. Honestly, I don’t have interest in probability and statistics, but recently I had to learn them to prepare my interview with Google. I have heard that they may ask this kind of questions.

Although I am not an expert in probability, I will try to explain how we get this number.

First, let’s start with a simple problem. We have 3 people in a room. What is the probability that two persons share the same birthday? Let’s take a date for the first person. The second person has probability of 1/365 to share the same birthday (we ignore leap year for simplicity). The third person also has the same probability, 1/365. We end up with 2/365.

Do we miss something? Indeed, yes. There is a probability that the second person shares the same birthday as the third person. Things are getting more complicated for 23 people.

How do we solve this problem then? We all know this formula from our class in high school:

P(two persons share the same birthday) = 1 - P(no two persons share the same birthday).

It’s easier to find the probability of no two person share the same birthday, isn’t it?

Back to our problem, let’s take a date for the first person from 23 people. The probability that the second person doesn’t share the same birthday is 364/365. The probability of the third person is 363/365. The probability of the fourth person is 362/365, and so on. So we end up in the following equation:

P(two persons share the same birthday) = 1 - (364 / 365) * (363 / 365) * … * (343 / 365)

P(two persons share the same birthday) = 50.7%

Update (28-Oct-06): Just additional note, Google didn’t ask me this question during interview. I just heard they may ask this kind of questions during the interview. Read this posting for an example.

you are at a party with a friend and 10 people are present including you and the friend. your friend makes you a wager that for every person you find that has the same birthday as you, you get $1; for every person he finds that does not have the same birthday as you, he gets $2. would you accept the wager?

For my case, they asked computing and programming questions. No brain teaser questions.

Two days ago, I posted a blog about my bad experience with Canadian doctor. I have been thinking why my problem is getting worse and worse So, I did some search on the Internet. Finally, I came to a conclusion that my problem was getting worse because of Hydrogen Peroxide (H2O2). Why? The “stupid” Canadian doctor recommended me to use Hydrogen Peroxide on my wound.

After searching information on the Internet, I found many interesting links about Hydrogen Peroxide. Here is a copy from EOpinions.com:

…I noticed throughout the week that she never used any Hydrogen peroxide on any wounds. She said not only does it bubble and eat away and the bad bacteria in the wound, it is also eating away and bubbling away good tissue! This means you are actually doing more harm that good…

This is another interesting post from a British nursery web site.

The problem with hydrogen peroxide and some other traditional debriding agents is that they also damage the healthy cells (keratinocytes and fibroblasts) that are needed for wound healing and inhibit their necessary migration into the damaged area

Lastly, the use of Hydrogen Peroxide is not recommended by the US Department of Health and Human Services as pointed out by TruthOrFiction.com.

The HealthFinder publication of the U.S. Department of Health and Human Services says don’t use hydrogen peroxide on a wound because it interferes with healing. The U.S. Gymnastics team has followed the recommendations of researchers and uses soap and water for cleansing wounds and not hydrogen peroxide.

So why did my doctor use Hydrogen Peroxide? Anyway, I have changed to another doctor now. I won’t come back to the previous doctor again. Never….

I have an advice for you: always find a good doctor. Many doctors don’t really know what they are doing, even in a country like Canada.

Recently I asked myself, why does North America use different system of measurement than the rest of the world? They measure length in mile, while the rest of us use meter. They measure weight in pound, while the rest of us use gram. They measure volume in gallon, while the rest of us use liter.

Basically we have two systems of measurement, i.e. metric system and English system. The metric system, a.k.a. SI (Système International d’Unités) is the most widely used system of measurement. The base units include meter and gram. The English system, a.k.a. British system, was originally introduced in the United Kingdom. They use familiar objects or parts of the body as the basic units of measurement. For example, distance on the ground is measured with foot. Volume is measured with gallon (the old name of pail is gallon).

When I moved to North America for the first time, I was a little bit confused. I am more familiar with SI and many people here are more familiar with English system. They say something like “It’s only five miles from here”, “The baby’s weight is 10 pounds” or “The area of this apartment is 800 square feet”. Although I had a lesson how to convert English system to SI in the high school, I have forgotten most of them.

Why does North America still use English system while the rest of the world uses SI? Actually the US government has required the use of SI in all business activities. Read more information at NIST web site. Unfortunately this mandate is easier to say than to realize. Americans are still using English system in their daily life.

How do we convert English system to SI (or vice versa)? The “no brain” way is to use Google Calculator. You can type the following expression in Google search box:

• 5 miles in meters
• 10 pounds in kilogram
• 800 square feet in square meter

I have also compiled a table of conversion between English system to SI:

• 1 inch = 2.54 centimeters
• 1 foot = 0.305 meter
• 1 mile = 1.609 kilometers
• 1 ounce = 28.35 grams
• 1 pound = 0.454 kilogram
• 1 (square foot) = 0.093 square meter
• 1 gallon = 3.785 liters
• 1 mile per gallon = 0.425 kilometer per liter

There is an interesting fact about 1 ounces. Some Indonesian people believe that 1 ounce = 100 grams. They even use this “standard” to buy something in the traditional market. Interesting, huh? In fact, when I was a kid, I was taught that 1 ounce = 100 grams. I don’t know the history and background of this. For all Indonesian people who read this blog, let’s tell our friends and relatives to correct this mistake.

Personally, I hope that Americans are willing to adopt SI. It’s too difficult to live with different standards with the rest of the world. Just an example in the mobile phones industry, Americans who use CDMA network for their mobile phones will find difficulties when they are traveling to Europe or Asia. The rest of the world is using GSM network, and many countries don’t support CDMA network.

Updated: I have comments from my readers that 1 ons in Indonesia is actually from Dutch measurement system. They use 1 ons = 100 grams. This is not the same as ounce in BTU. Thanks to Supique and Tedja for the comments.