8. 
Which statement is untrue? 

The "Relative frequency diagram" represents the number of failures within time
intervals, i.e. the failure distribution over time. 

The "Probability density function" of failure represents the number of failures
within time intervals divided by the total number of failures, i.e. the
probability of failure density over time. 

The "Normal distribution" also called the "Gaussian distribution" is the most
widely used probability density distribution. 

Other probability density distributions are the "Wear out", "Bathtub", "Random"
and "Infant mortality" functions. 
