Generally, the five largest lichen thalli diameters are taken, although several statistical methods have been used. If the growth rate is known, the maximum lichen size will give a minimum age for when this rock was deposited. The growth rate curve, a graph of age of a lichen against the date of the substrate on which it is found has to be constructed for an area.
Typically, the five largest lichen thalli are measured; various statistical techniques have been used. If the growth rate is known, the maximum size will indicate an approximate age when the rock was deposited. A graph showing the relationship between a lichen's age and the date of its substrate must be created for a specific area.