TI-83 Instructions

Computing a Normal Probability

Let X be a normal random variable with mean m and standard deviation s.

The three basic forms:

P(X < a)

P(X > a)

P(a < X < b)

Normal Probability P(X < a)

Method 1

1. Press 2^nd DISTR. The DISTR menu title is highlighted.
2. Select normalcdf, the item #2, or press 2 and skip the next step.
3. Press ENTER. The phrase normalcdf( appears in the display.
4. Enter a very large negative number, e.g., -1000000.
5. Press comma.
6. Enter the value of a.
7. Press comma.
8. Enter the value of m.
9. Press comma.
10. Enter the valueu of s.
11. Press ).
12. Press ENTER. The value of P(X < a) appears in the display.

Method 2 (z-score)

1. Enter the value of a.
2. Press – (minus).
3. Enter the value of m.
4. Press ENTER. The value of the deviation a – m appears in the display.
5. Press / (divide). The expression Ans/ appears.
6. Enter the value of s.
7. Press ENTER. The z-score (a – m)/s appears.
8. Compute P(Z < z-score) as a standard normal probability.

Normal Probability P(X > a)

1. Compute P(X < a).
2. Subtract the value of P(X < a) from 1. The result is P(X > a).

Normal Probability P(a < X < b)

1. Compute P(X < b).
2. Compute P(X < a).
3. Subtract the value of P(X < a) from P(X < b) to get P(a < X < b).