Google is celebrating its seventh birthday and has expanded its index to make it three times larger than its nearest competitor's. But Google-watchers are left in the dark over the total number of pages being searched.

Previously listed as eight billion or so, the number has now been removed. Chief executive Eric Schmidt explained that "people do not necessarily agree on how to measure it".

The search giant now claims that its index is three times larger than any other search engine's "in terms of unduplicated pages".

"Google opened its doors in September 1998 and has been pursuing one mission ever since: to organise the world's information and make it universally accessible and useful," said Google software engineer Anna Patterson in her blog. 

"For our seventh birthday we are giving you a newly expanded web search index that is 1,000 times the size of our original index."

Last month Yahoo claimed that, at more than 20 billion, its index is larger than Google's, a claim denied by Google.

Search experts have maintained that relevance is of more value to users than the sheer size of the index. But owing to the proprietary nature of search algorithms, there seems little chance of any standardisation in the technology of relevance.

Google has also unveiled an option to search using a specialised query to see which of a number of searches provides the most results.