Dear rotiboy,
Well, IMEX, it depends on yr internal decision-making plan.
For example, when you source raw materials etc, one usually does not make a "stop" decision on a regular supplier based solely on one lot unless there is some horrific disaster (eg quality, trust or money-wise).
An example of "horrific" might be if this were a new supplier and business was initiated on the basis of a trial sample which was of a totally different calibre to the clearly unacceptable delivered lot. It happens. 
In contrast, if you hv been doing regular business for years with no comparable problems so that this was probably caused by some abnormal circumstances, the reaction would usually be different.
A generic approach is to set up an action system based on a mixture of safety (Ma/Mi/Cr etc) / non-safety defects. The former criteria tend to be assessed continuously while the latter are time-averaged, graded, and acted on accordingly (eg Grade A max 2% defects / month = satisfactory, D = >10% = Unsatisfactory = action). Similar set-up as used by factory auditors and examples of which are scattered around this forum (eg try search for "supplier"). Usually necessary to hv a separate evaluation for new suppliers.
The ease of application may depend on the product form / quantity / value / number of lots per time interval. Some official sets of plans exist, eg Mil. Standards.
Rgds / Charles.C
