How did ecologists estimate the total number of species on Earth?

(N/A) The estimate of the total number of species on Earth was provided by the $IUCN$ $(2004)$ report. According to this,the total number of plant and animal species described so far is slightly more than $1.5$ million.
Many species are yet to be discovered and described. Most of these are based on estimates.
Temperate regions are more thoroughly studied for taxonomic groups and species discovery compared to tropical countries.
$A$ significantly large number of species remains to be discovered in the tropics.
Biologists have extensively studied the species richness of insect groups in tropical and temperate regions. By making statistical comparisons between these groups and extrapolating them to other groups of animals and plants,they have estimated the total number of species on Earth.
While some estimates range from $20$ to $50$ million,the most scientifically sound estimate made by $Robert$ $May$ places the global species diversity at about $7$ million.