Conditional Probability Activity
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Teacher Demo for the Class or Class Activity
as an Introduction to Conditional Probability

 Teacher Demo:  Prepare a small bag with colored slips of paper, or colored candies, or colored paper clips, or etc.  Show the students what is in the bag.  List the information on the board or overhead.

Ask the students to predict the probability of drawing out two items of the same color in a row
•  if you draw both items with your eyes closed (so you do not see what you get on the first draw)
• if you draw the first item, look at it, and then draw the second item.

In each case, discuss your knowledge of what remains in the bag after the first draw.  Students need to understand that the probability of drawing a second item changes when you have partial information from the first draw.  Seeing an example will help to clarify this often confusing process.

 

Class Activity:  This same idea can be used for a class activity.  Small Halloween candy bags work nicely.  Place colored slips of paper in the bags (or colored wrapped candies).  Hand out one bag per pair of students.  Ask students to empty the bag and examine (but not eat) the sample space.  Return the items to the bag to begin the activity. 

The query:  what is the probability of drawing out two items of the same color (kind) in a row?

Have students randomly draw one item from the bag.  Then have them discuss the probability of drawing a second item of the same kind from the bag, under two conditions:
•  they did NOT look at the first draw, or
•  they DID look at the first draw. 

When students have had a chance to discuss, ask for solutions.  Hold a class discussion about the experiment.