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Intro to Probability
Common Core Standard: 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.
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Probability- is the chance that something will happen
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Event- is the something that you are trying to determine the probability of
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Experiment- for probability, the experiment is the action that you are undertaking
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Trial- one run of an experiment
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Outcome- is the result of a single trial of an experiment
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Favorable outcome-is the outcome that matches the probability that you are trying to determine (this does not
mean that it is a happy occurrence)
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Sample space is the list of all possible outcomes
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Theoretical Probability- what the probability should be in theory
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Experimental or Actual Probability- what the probability is after conducting an experiment
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The way probability is determined is by dividing the favorable outcomes divided by the total number of outcomes
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This means that probability is always a number between 0 -1
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Probability can be represented as a fraction in lowest terms, a decimal, or a percentage
Example: What is the theoretical probability that a regular 2-sided coin will land on heads?
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The event is what is the probability of getting a heads
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The experiment is the actual flipping of the coin
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The outcome is 2, because there are 2 possible choices for the coin to land
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The sample space is heads or tails
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Probability is ½, since there is only 1 outcome that is favorable out of the two possible outcomes
Probability Scale:
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This is a tool that shows the range of probabilities and what each represent
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It is used for theoretical and experimental probability
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If an event has a 0 probability this means that it can never happen, because the favorable outcomes is zero
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If an event has a 1 probability it will certainly happen, because there is the same number of favorable
outcomes as there is total number of outcomes
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If an event has a 0.5 probability, it has an equal chance of happening or not happening (flipping a coin)
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If an event has a probability between 0 and 0.5, then it is unlikely to happen, but not impossible
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If an event has a probability between 0.5 and 1, then it is likely to happen, but not certain
Example: Determine each event’s below theoretical probability by using the probability scale
a) Probability of rolling an even number on a regular 6-sided die
b) Probability of rolling a 5 on a regular 6-sided die
c) Probability of rolling a 9 on a regular 6-sided die
d) Probability of rolling a number 1-6 on a regular 6-sided die
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The probability for a is ½ or equally likely to occur, since there are 3 even numbers out of 6 total outcomes
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The probability for b is or unlikely to occur, since there is only one 5 out of the 6 total outcomes
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The probability for c is 0 or impossible, since 9 does not appear on a regular 6-sided die
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The probability for d is 1 or certain, since the numbers 1-6 are the only numbers present on the die
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