Hi, my name is Bassem Saad. I'm a Math Ph.D. candidate at U.C. Davis, and I'm here today for About.com to answer the question: “What is probability?”Probability is a mathematical way of measuring the likelihood or frequency of an event. Now these events could be anything from rolling a four on a six-sided dice, or creating a pair of subatomic particles from one single photon. There's no way of knowing if either of these events will occur, but with probability, you can get a pretty good idea of how likely they are to occur.Probability is measured by percentage. The higher the percentage the more likely it'll occur, while the lower the percentage the less likely it'll occur. The highest percentage an event can have is 100 percent, which means it's definitely going to occur. The lowest percentage it can have is zero percent, which means it will never occur.Let's take a closer look at a fair six-sided dice -- “fair” just means any number on the dice is just as likely to occur. There are six sides on a six-sided dice, so you could roll a one, two, three, four, five, or six. Say you want to measure the likelihood of rolling a four. Well, four is just one possibility out of six; that is, one to six, or about 17 percent. Now let's say we're interested in measuring the likelihood of rolling either a three, a five, or a six. Well, that's just three numbers out of the possible six, so we have a ratio of three to six, or one to two, which is really just 50 percent.The mathematics of probability has had a great deal of success in helping scientists understand the world around us. Truly, modern physics could not exist without probability. And medical researchers need probability to measure the effectiveness of new drugs. Thanks for watching, and to learn more, visit us on the web at About.com.

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