The h-index is used to gauge a researchers' productivity. If Dr. Amar has published five papers with the following citation counts : 21, 12, 4, 2 and 1, then what is the h-index of Dr. Amar ?

Ranking the citation counts as 21, 12, 4, 2, 1 shows three papers each cited at least three times, so the h-index is three.

The fourth paper has only two citations, which is below four, so an h-index of four fails.

An h-index of five would need every one of the five papers cited at least five times, which the low counts of 4, 2 and 1 rule out.

The value ten simply doubles the count of papers and has no basis in the h-index rule.

The h-index balances quantity of papers against citations each has received.

Physicist Jorge Hirsch proposed the h-index in 2005.