 When "z" is positive, the tail probability is 1 − ℙ("y" ≤ "z").Because normal distributions are symmetric, the upper and lower tail probabilities will be equal, and thus you can find the upper probability and multiply by 2 to find the combined tail probabilities.If "z" is negative, find the negative tail probability, ℙ("y" ≤ "z"), and multiply by 2 to find the combined probability in both upper and lower tails.