If the results are not within acceptable limits of variability, determine where something could have gone wrong. Common problems include not mixing the sample well enough prior to analysis, not measuring accurately, and incorrect incubation temperature. Briefly, you should maintain records of positive and negative controls, incubator temperatures, and split sample results. Maintaining proof that your results were generated in a consistent, reproducible manner that adheres to the requirements of the method will allow others to accept your results. Quality control testing shouldn't take too much extra time, but it will instill confidence that you are producing valid data.
Different methods have different precision and recovery ability.
It is important to separate results that were generated by different test methods and under different conditions. Gerri A. Miceli is a Water Resources Specialist at the Arizona Department of Water Resources.