The purpose of External Quality Assessment is to compare performance of individual laboratories or test centres with their peers. EQA is a retrospective form of quality control - participants do not know the target range or expected results for their test, unlike internal quality control.
Performance is determined by comparison of individual laboratory results with the target value for each test. UK NEQAS BC uses median values determined from participants' results to determine the consensus or "target" values against which individual laboratory performance can be assessed. Use of the median avoids the effect of outlying results and the need to remove any outlying data from the analysis.
Results are assessed using statistical measures comparing individual results with peer group or overall consensus values. Therefore, a result falling outside the statistically defined terms is not necessarily a "poor" or "unsatisfactory" result. We deliberately employ the term "outwith consensus" to indicate a result that falls outside the statistically determined limits we have applied.