SKEDSOFT

Introduction:

Reliability is the probability than an item can perform its intended function without failure for a specified interval under stated conditions.

Description:

Reliability is the probability that a system will still be functioning at time t.

This can be expressed as “the cumulative distribution of failure”

These two measures are the mirror image of each other (Refer Figure below). The reliability will start at 1 and decay to approach 0 over time. The cumulative distribution of failure will start at 0 (no failures) and approach 1 as all the items fail over time. The slope of the reliability curve at any time t is the failure rate at that point in time. These measures give the overall reliability or failure at time t

Probability density function

We wish to have an idea of the probability of an item failing in a given unit time period. This is termed the “probability density function” and is given by

The failure or hazard rate gives the failure density over a period of time as with the “probability density function”, but is based on the current population. This gives a much better indication of the changing reliability of a system over time.