Three variables are used to work out the Carlson Index: "chlorophyll pigments", "total phosphorus" and "Secchi depth". Chlorophyll is the most accurate measure, as it is the best predictor of biomass. Phosphorus may be better in summer. Secchi depth is the least accurate measure, but it is the cheapest and easiest to use. It measures how clear the water is. This shows how much dissolved and particulate material is in the water. This can be used to work out the biomass.