Hi Nuttawut,
for assessment you need some more information e.g. the sampling plan etc.
example:
Salmonella-"free" is often written as n.d. in 25 g
The result can be based on
a) one sample of 25 g is taken from a large batch
b) 30x 25g samples are taken in a representative mode from the same batch -> is often written as n.d. in 25 g too
(we have performed such sampling by automatic valve during loading e.g. for chocolate)
From statistical point of view there is a huge difference for a risk assessment.
Example b is sometimes written as n..d in 750 g. But it will not become clear whether the result is based on one sample of 750 g or 30 samples of 25g.
The better way to write e.g. micro specs as n, c, m , M as described here on p 15
microbiological-specifications-nestle.pdf
Rgds
moskito