Common Core Standards: Math
7. Understand that the probability of a chance event is a number between 0 and 1 that expresses the likelihood of the event occurring. Larger numbers indicate greater likelihood. A probability near 0 indicates an unlikely event, a probability around 1/2 indicates an event that is neither unlikely nor likely, and a probability near 1 indicates a likely event.
Chances are that before seventh grade, students haven't had a lot of exposure to probability. They've probably heard words like, "chance," "odds," and, well, "probability" thrown around and, using context clues, pieced together a kind of Franken-understanding of what probability is without ever really knowing the basics. Well, it's time to set 'em straight.
Students should understand that the probability of an event is a measure of how likely it is that the event will occur. It's always expressed as a number on a scale from 0 to 1, with 0 meaning the event will never occur and 1 meaning the event is guaranteed to happen. Like us winning the lotto, right?
Since we're looking at a scale from 0 to 1, students should also reason that if the probability of an event is close to 0, it's unlikely to happen, and if it's close to 1, it's likely to happen. The number is smack-dab in the middle of 0 and 1, which means the event is neither unlikely nor likely. Or equally likely and unlikely.
Make sure students understand other basic elements of probability, like the fact that probabilities mean nothing unless they're linked to specific events. A probability of 0.1 is all fine and dandy, but what is it a probability of? Without an event, there's a probability of 0 that a probability of 0.1 makes any sense at all.
As far as this standard is concerned, all students need to do is understand, so there's no need for probability calculations just yet. All we want is for students to know how the probability scale works—that larger numbers (i.e., numbers closer to 1) mean that the event is more likely, and that smaller numbers (i.e., numbers closer to 0) mean that the event is less likely. That's it.
It's also not a bad idea to start developing informal ideas of probability in terms of actual events like rolling a die, tossing a coin, and playing the lotto. (Just kidding. Hopefully.) They don't need to understand the calculation of probability just yet; all they need to do is reason that since neither heads or tails is more likely than the other to occur, each has a probability of Either way, heads we win, tails you lose.
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