Basic Math & PreAlgebra AllinOne For Dummies (+ Chapter Quizzes Online)
Explore Book Buy On AmazonFor two independent events, here’s how to use the probability square:

Draw a rough square.
Mark the topleft corner with 0, and the topright and bottomleft corners both with 1. (Don’t bother with the other corner.)

On the horizontal probability line, mark the probability of the first event.
You don’t have to measure the line. Just put a cross somewhere that looks reasonable.

Draw a line down the page from your cross to the opposite edge of the square.

Label the horizontal probability line with the probability (to the left of the line) and the ‘what’s left over’ (one take away the probability) on the right.

On the vertical probability line, mark the probability of the second event.
Don’t measure – just estimate and draw a small cross.

Draw a line across the square from the small cross.
Your diagram should now look like a big cross. The topleft rectangle represents the probability of both events happening; the topright rectangle shows the probability of the second event happening but not the first; the bottomleft rectangle gives you the probability of the first event happening but not the second; and the bottomright rectangle gives you the probability of neither event happening.

Label the vertical line with the probability (above the line) and the number left over (below it).

To find the probability of both things happening – the topleft quarter – you times the number at the top of the column by the number at the left end of the row.
Remember, to get the probability of two events occurring, you have to times them together. Visually, this corresponds to the area of the rectangle, so if you were to draw it accurately the biggest areas would represent the largest probabilities.

To find the probability of any combination of events, find the correct area (as described in Step 6) and times the number at the top of the column by the number at the left end.