Topics
8. Probability
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United States of America
Grade 7

Investigation: Develop a probability model

Lesson
Practice
Overview
Activity 1
Activity 2
Reflection

Investigate theoretical probability and the concept of convergence.

Objectives
  • To understand the similarities and differences between theoretical and experimental probability.
  • To simulate trials.
  • To understand the concept of convergence.
Materials
  • Coin
  • Standard dice (optional)

Dice experiment

Experiments
Some ideas or theories need to be confirmed through experiments before determining if they are true or not. For example, if scientists believe they have found a new vaccine, they need to test it to check if it does indeed work. In mathematics, we will often have a hypothesis, test it with a few numbers to check, and then prove it mathematically using other known facts.
How many trials?
Let's go back to the example above of testing a vaccine. How many trials do you think the scientists would do before deeming the vaccine safe? One, five, one hundred, one thousand? They would want reliable and consistent results before making a conclusion.
Use the applet below to complete the following steps:
  • Roll the die 6 times by clicking the "Roll Once" button. Do you get one of each value? If not, what is the relative frequency (experimental probability) for each value?
  • Roll the die another 4 times by clicking the "Roll Once" button and then use the "roll \times 10" button to get to 60 rolls. Do you get 10 of each value? If not, what is the relative frequency (experimental probability) for each value?
  • Use the "Animation" or "roll \times 10" buttons to see what happens to the experimental probabilities as the number of trials increases.
Loading interactive...
Investigate
Consider the following questions once you have completed the above procedure.
1.
What is the theoretical probability of rolling a 5 on a standard die as a fraction, decimal and percent?
2.
If you roll a standard die 60 times, how many times would you expect to roll a 2?
3.
If you roll a die 1500 times, how many times would you expect to roll a 3?

Coin experiment

Fun fact
In the movie, The Dark Knight, one of the main characters has a coin with a heads on both sides and claims that he "makes his own luck." While we might likely notice if someone had a coin with two heads, there is another trick which is weighting one side of the coin, so it is more likely to land down on that side.
If a coin is indeed fair, the experimental probability of flipping heads and tails should both be about 50\%. As a class, see how closely the experimental probability is to the theoretical probability of 50\%.
Complete the following procedure:
  • With a partner or independently, flip a coin 20 times making a tally of how many times it lands on head and tails.
  • Combine the results for the whole class to find the total frequency of heads, tails and number of trials.
Investigate
Consider the following questions once you have completed the above procedure.
1.
What is the experimental probability of heads and tails from your 20 coin flips?
2.
What is the experimental probability of heads and tails for the combined results of the class?

In the two completed activities, we see the idea of convergence. This means that as the number of trials gets very large, the experimental probabilities will begin to get closer to the theoretical probabilities. The idea with convergence is that if we continued to roll the die forever, eventually the experimental probability would stop changing. The value it will settle on would be the theoretical probability.

Discussion
1.
Fill in the blanks in the following sentence. As the number of ⬚ gets very large, the experimental probability approaches the ⬚.
2.
Can you ever be 100\% confident in a hypothesis? Explain why or why not? Are there some scenarios where you can be and other where you can't be?
3.
How might the idea of more trials being more accurate fit into other scenarios?
4.
There is a balance between doing trials and the cost of the trials. Let's go back to the first experiment of rolling a die. At what point might we say, "we're confident that this die is fair" and stop rolling? go back and experiment with the applet a couple of times and pause it (bottom left corner) when you think it is good enough.

Outcomes

7.SP.C.5

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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