Colourimetry Errors?

[VCEMan94]

What are some errors that could account for an under and overestimation?

[Christiano]

-The non-homogenous nature of the mixture - your sample may contain less concentration of phosphorus and there a lower % and vice versa.
-Some solution still stuck on the filter paper (if used) or stirring rod = lower %
-Some insoluble compounds may be in the solution, so the phosphorus as well as the insoluble compounds will absorb the light and give higher absorbancy = higher %