The tutor gives an example of a confidence interval.

Example 1:

Let’s imagine a part has mean lifetime thirty-six months, with standard deviation ten months. What minimum time will it last, with 95% confidence?

Solution:

Let’s use μ for mean, and σ for standard deviation.

From the normal table, 95% of the distribution is less than μ + 1.645σ.  By symmetry, 95% is more than μ – 1.645σ.  Therefore, 95% of the time, the part will last more than
36 – 1.645×10 months, or 19.55 months.

Note this confidence interval makes no guess at the top of the life span.  I’ll talk about other confidence intervals in future posts.

HTH:)

Source:

Harnett, Donald L. and James L. Murphy. Statistical Analysis for Business and
   Economics. Don Mills: Addison-Wesley, 1993.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.