Global effective degrees of freedom

It can be obtained form Welch-Satterthwaite formula:

is the effective degrees of freedom of PDF associated

with

- if evaluation of standard uncertainty of is of type A,

is degrees of freedom of corresponding Student's t-distribution

(number of observations -1 if it's the average of repeated independent

observations)

- if it's type B evaluation et we don't have specific information,

may be approximated by:

where is the relative uncertainty of

.