FIT Rates

h(t) = hazard rate = f(t)/R(t)
Hazard rate is instantaneous failure rate. But usually, average failure rate for a given time period is more useful, for eg. failure rate per hour.

FIT rate is defined as ppm failure per 1000 hours or number of fails per 10^9 device hours.

Multiply hourly fail-rate by 10^9 to generate FIT rate.

Hourly fail rate = No of rejects / (No of devices x No of hours x AF)

FIT rate = Hourly fail rate x 10^9

No of rejects is determined by Chi-square distribution = [(x^2)/2], commonly at 60% confidence (alpha) and dof: 2r + 2