Set up a table in which the values for one variable are displayed across the top of the table, and the values for the other variable are displayed to the left of the table. For example, if you were looking at the probabilities that a frog was green or brown, and preferred to eat crickets, moths, or mealworms, then you would list the two colors along the left side of the table and the three foods along the top of the table.
Insert the probabilities for each value along the side of the table opposite where the values of the variable are listed. If the probability that the frog is green is 70 percent and the probability that it is brown is 30 percent, then write 0.7 on the right side of the table directly opposite the word green, and 0.3 on the right side of the table opposite the word brown. If the probability that a frog showed a preference toward crickets, moths, or mealworms is 40 percent, 35 percent, and 25 percent respectively, then write 0.4, 0.35, and 0.25 under the graph beneath the corresponding food.
Multiply the probability of each value for one variable times the probability of each value for the other value, and write the product of the two probabilities into the intersecting box of the table. Since 70 percent of the frogs are green and 40 percent prefer to eat crickets, multiply the two probabilities together and write 0.28 at the intersection of green and cricket. The other probabilities in this example should be 0.245, 0.175, 0.12, 0.105, and 0.075.
Add the probabilities of each intersecting box of the table together. If the computations have been done correctly, the sum of the joint probabilities should equal one.