Content

• Summarize in 10 seconds
• Steps
• Advice

Probability is a measure of the likelihood that an event will occur out of the total number of possible outcomes. Through this article, wikihow will help you learn how to calculate different types of probability.

Summarize in 10 seconds

1. Identify events and outcomes.
2. Divide the number of events by the total number of possible outcomes.
3. Multiply the result in step 2 by 100 to get the percentage value.
4. The probability is the result calculated as a percentage.

Steps

Part 1 of 4: Calculate the probability of a single event

1. 

   Identify events and outcomes. Probability is the probability that one or more events will occur out of the total possible outcome. So, for example, you are playing dice and want to know the possibility of shaking the number 3. "Shake the number 3" is the event, and as we know a dice has 6 faces, therefore, The total number of possible outcomes is 6. Here are two examples to help you better understand:
   • Example 1: When choosing any day of the week, how likely is that the weekend falling?
     • Choose a date that falls on the weekend is an event in this case, and the total probable outcome is the total number of days of the week, ie seven.
   • Example 2: A jar contains 4 blue marbles, 5 red marbles and 11 white marbles. If you take any one stone from the jar, what is the probability that you will get the red marble?
     • Select a red stone is the event, the total number of possible outcomes is the total number of stones in the bottle, ie 20.

2. 

   
   
   Divide the number of events by the total number of possible outcomes. This result tells us the probability that a single event is likely to occur. In the case of the dice above, the number of events is one (there is only one side 3 out of the total of 6 sides of the dice), and the total number of possibilities is 6. So, we have: 1 ÷ 6, 1/6, 0.166, or 16.6%. For the remaining examples, we have:
   • Example 1: When choosing any day of the week, how likely is it to fall on the weekend?
     • The expected number of events is two (since the weekend consists of two Saturdays and Sundays), a total of seven possibilities. So the probability that the selected date falls on the weekend is 2 ÷ 7 = 2/7 or 0.285, equivalent to 28.5%.
   • Example 2: A jar contains 4 blue marbles, 5 red marbles and 11 white marbles. If you take any one stone from the jar, what is the probability that you will get the red marble?
     • The number of possible events is five (because there are 5 total of those colored stones), the total number of possible outcomes is 20, which is the total number of stones in the jar. So the probability of choosing a red stone is 5 ÷ 20 = 1/4 or 0.25, equivalent to 25%.
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Part 2 of 4: Calculate the probabilities of many events

1. 

   
   
   Divide the problem into many small parts. To calculate the probabilities of many events, the main thing we need to do is to break the whole problem down into terms individual probability. Consider the following three examples:
   • Example 1:What is the probability of rolling the dice 5 twice in a row?
     • We already know that the probability of shaking face 5 in each roll of the dice is 1/6, and the probability of shaking face 5 in each roll is also 1/6.
     • These are the independent event, because the result of the first roll of the dice does not affect the result of the second; ie the first time you shake face 3, the second time you can still shake face 3.
   • Example 2: Randomly draw two cards from a deck of cards. How likely is the chance to draw two leaves of the same shrimp (or shrimp or dragonfly)?
     • The chance that the first card is a play is 13/52, or 1/4. (There are 13 cards in each deck of cards). Meanwhile, the chance that the second card is also a clo is 12/51.
     • In this example, we are looking at two dependent event. That is, the first result has an impact on the second time; for example, if you draw a 3-card and do not re-insert this card, the total number of cards remaining in the deck will be reduced by 1, and the total number of cards will be reduced by 1 (i.e., 51 leaves instead of 52).
   • Listing 3: One jar contains 4 blue marbles, 5 red marbles and 11 white marbles. If 3 stones are taken out randomly, what is the probability that the first stone is red, the second marble is blue and the third is white?
     • The probability that the first stone is red is 5/20, or 1/4. The probability that the second stone is blue is 4/19, because one stone has been reduced in the jar, but not a colored stone. blue. The probability that the third marble is white is 11/18, since we have removed two non-white stones from the bottle. Here is another example of dependent event.

2. 

   
   
   Multiply the probabilities for single events. The product obtained is the combined probability of the events. As follows:
   • Example 1: What is the probability of rolling the dice 5 twice in a row? The probability of each independent event is 1/6.
     • So we have 1/6 x 1/6 = 1/36, which is 0.027, which is 2.7%.
   • Example 2: Randomly draw two cards from a deck of cards. How likely is the chance to draw two leaves of the same shrimp (or shrimp or dragonfly)?
     • The probability that the first event happens is 13/52. The probability of the second event occurring is 12/51. So the combined probability would be 13/52 x 12/51 = 12/204, or 1/17, or 5.8%.
   • Listing 3: One jar contains 4 blue marbles, 5 red marbles and 11 white marbles. If 3 stones are taken out randomly, what is the probability that the first stone is red, the second marble is blue and the third is white?
     • The probability of the first event is 5/20. The probability of the second event is 4/19. The probability of the third event is 11/18. So the combined probability is 5/20 x 4/19 x 11/18 = 44/1368, or 3.2%.
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Part 3 of 4: Convert odds ratio to probability

1. 

   Determine odds ratio. For example, the odds for a golfer to win is 9/4.The likelihood ratio of an event is the ratio between its probability will happened compared with the probability that the event are not happening.
   • In the 9: 4, 9 example example represents the probability that the golfer will win, while 4 represents the probability that the golfer will lose. Therefore, the probability of this golfer winning is higher than the probability of losing.
   • Remember that in sports betting and bookmaking with bookmakers, odds are usually expressed in terms odds ratio, that is, the rate at which the event happened is written first, and the rate of the event not happening is written later. This is a point to remember because such writing is often misunderstood. For the purposes of this article, we will not use such inverse odds ratio.

2. 

   Convert probability ratio to probability. To convert probability ratios to probabilities is not difficult, we just need to convert the odds of probability into two separate events, then add up the probability to get the total possible outcome.
   • The event that the golfer wins is 9; the event that the golfer loses is 4. So the total probabilities are 9 + 4 = 13.
   • Then we apply the same calculation as the probability of a single event.
     • 9 ÷ 13 = 0.692 or 69.2%. The probability that the golfer will win is 9/13.
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Part 4 of 4: Rules of probability

1. 

   Make sure that the two events or outcomes need to be completely independent of each other. That is, two events or two outcomes cannot happen at the same time.

2. 

   Probability is a non-negative number. If you find out the probability is a negative number, you need to check your calculation.

3. 

   The sum of all possible events should be 1 or 100%. If this sum is not equal to 1 or 100%, you missed an event somewhere, leading to false results.
   • The ability to shake a face 3 when shaking a 6-sided dice is 1/6. But the probability of shaking in one of the other aspects is also 1/6. We have 1/6 + 1/6 + 1/6 + 1/6 + 1/6 + 1/6 = 6/6 or 1 or 100%.

4. 

   An event that cannot occur has a probability of 0. That is, the event is not likely to happen. advertisement

Advice

• You can build a probability based on your opinion about the likelihood of an event happening. The probability of conjecture based on personal opinion will differ from person to person.
• You can assign numbers to events, but they need to have an appropriate probability, that is to follow the basic rules of statistical probability.