# TI-83 Instructions

## Computing a Normal Probability

Let X be a normal random variable with mean μ and standard deviation σ.
- The three basic forms are:
- P(X < a)
- P(X > a)
- P(a < X < b)

- Normal Probability P(X < a)
- Method 1
Press 2nd DISTR. The DISTR menu title is highlighted.
Select normalcdf, the item #2, or press 2 and skip the next step.
Press ENTER. The phrase normalcdf( appears in the display.
Enter a very large negative number, e.g., -1000000.
Press comma.
Enter the value of a.
Press comma.
Enter the value of m.
Press comma.
Enter the valueu of s.
Press ).
Press ENTER. The value of P(X < a) appears in the display.
Method 2 (z-score)
Enter the value of a.
Press – (minus).
Enter the value of m.
Press ENTER. The value of the deviation a – m appears in the display.
Press / (divide). The expression Ans/ appears.
Enter the value of s.
Press ENTER. The z-score (a – m)/s appears.
Compute P(Z < z-score) as a standard normal probability.
Normal Probability P(X > a)
Compute P(X < a).
Subtract the value of P(X < a) from 1. The result is P(X > a).
Normal Probability P(a < X < b)
Compute P(X < b).
Compute P(X < a).
Subtract the value of P(X < a) from P(X < b) to get P(a < X < b).