Percentiles
main topic 

At what stress level do half of the units fail? How much pesticide do you need to apply to kill 90% of the insects? You are looking for percentiles.

Common percentiles used are the 10th, 50th, and 90th percentiles, also known in the life sciences as the ED 10, ED 50 and ED 90 (ED = effective dose).

The probit analysis automatically displays a table of percentiles in the Session window, along with 95% fiducial confidence intervals. You can also request:

·    additional percentiles to be added to the table

·    normal approximation rather than fiducial confidence intervals

·    a confidence level other than 95%

The Percentile column contains the stress level required for the corresponding percent of the events to occur.

In this example, you exposed light bulbs to various voltages and recorded whether or not the bulb burned out before 800 hours.

Table of Percentiles

As shown in the first and second columns of the first row, at 104.9931 volts, 1% of the bulbs burn out before 800 hours.