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Unit III Lesson 3

Introduction to Probability


Here is a colorful explanation of basic probability with nice explanations of related terminology. Note that a probability is expressed as a number from 0 to 1. If an event has a probability of 1, it is a sure thing!  A probability of zero? Don't bet on the event happening! 

Probability can be expressed as a fraction, or a decimal, or a percentage; you know how to convert from one to the other by now. Use the toggle key on your calculator if available.

Start simple here, watch a more complicated example here and here and then try out some simple problems. Make sure you can do these before going ahead. Scientists use probability all the time when they do genetic experiments.

It gets more complicated with predicting more than one event.  These are problems where the question is what are the chances that BOTH events happen, and then you need to ask if the events are independent of each other or dependent.  Watch these videos, one and two to start thinking about compound events.  You can watch the lessons starting here about compound probability of independent events and then watch compound probability of dependent events.  The definitions are easy to understand but can be tricky to apply; try out some independent and then the harder dependent word problems.

All the above problems involved multiplying probabilities but sometimes you need to add probabilities for two events. The Math Doctor at Drexel gives an explanation of when you add and when multiply probabilities. Here is some more guidance on when to use probability with addition and probability with multiplication, with questions and answers to test yourself.  

You can practice solving more probability problems at the regents prep site (note some of questions are actually about combinations, covered in next lesson).

Last statistics topic, lesson 4 on combinations and permutations, "how many ways?"

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