Organism counts from environmental quality control (QC) samples, such as indicators in water, will show variation even if they come from a single well mixed batch. This is due to random variation and there may be additional effects from organism behaviour. Thus the samples are not homogeneous in their content. Samples for chemistry will also be variable in their content but this may be at a level below the limit of detection and thus the observed measurements will show apparent homogeneity.
Organism counts may take 24 hours to achieve and longer to confirm. A chemical result can often be obtained rapidly. Assessment of performance with microbiological counts usually requires the accumulation of evidence over several samples. A judgement can be attempted with each chemical QC result.
QC in environmental microbiology is an important and growing field. The apparent imprecision and slowness in assessing performance is unavoidable. Sound quality assurance programmes will accommodate these factors.