Estimating the precision of serial dilutions and colony counts: contribution of laboratory re-calibration of pipettes

The basic (inherent) precision of serial dilutions and of colony counts made from them may be reliably estimated by reference solely to the pipette-manufacturerʹs specifications. Such estimates do not include external sources of variation and may be regarded as minima. The quality of estimation can be improved by using information gained by laboratory (‘in-house’) re-calibration of the pipettes. The degree of improvement was assessed by comparison with similar series made without re-calibration. It was found that improvement was minimal for colony counts but worthwhile for homogeneous solutions.