Terminology, concepts and definitions - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 24

1. 10 Evaluation of c.d.f..

1. 11 Evaluation of p.d.f..

ˆ

P
A t T S

K

￿ ￿ ￿ ￿ ￿

∑

ˆ

ˆ

3

= ( ) −

i

;

ˆ

i 0

3

1

- sampling excess operating time to failure

ˆ

 

K

P

∑

ˆ

ˆ

4

E = ( − t T S )

i

− 3.

 

i 0

ˆ

4

 

1

These characteristics are used to select the approximating function.
So, the asymmetry coefficient is a characteristic of 'skewness' of the
distribution, for example, if the distribution is symmetric with respect to
the EV, then A = 0.
In Fig. 1.13a, the distribution f 2 ( t ) has a positive asymmetry A > 0 and
f 3 ( t ) negative A < 0.
The excess characterises the 'steepness' (sharp or flat-tipped) of the
distribution. For the normal distribution E = 0.