How clicks are distributed in Google's result pages?

What is the distribution of clicks on links in the SERPs, depending on the position in the page?

A statistical study by a university on the one hand and a leaked document from AOL on the other give a first answer, at least 70% of clicks are on the first three links and about 50% over the first.

The statistical study was conducted by Cornell University at Ithaca in the New York state. It leads to the following table:

Ranking in SERPs Clicks % Time spent %
1 56,36 28,43
2 13,45 25,08
3 9,82 14,72
4 4 8,7
5 4,73 6,02
6 3,27 4,01
7 0,36 3,01
8 2,91 3,68
9 1,45 3,01
10 5,55 2,34
Total 98,9% 99%

This study was conducted on a population of students who have completed 397 requests. This does not represent the online population and covers only the first page or results.

Another answer was given to us when the statistics of the AOL search engine have been made public by mistake, it is probably closer to the truth because on a population of millions of Internet users.
It always depends on the nature of queries on the one hand, titles and descriptions on the other:

Ranking % of clicks
1 47%
2 13%
3 9%
4 7%

This is 70% for the first three links, the study returns 80% but only on the first page.

A study published by Chikita Insight on May 25, 2010 show different results again:

Ranking % of clicks
1 34%
2 17%
3 11%
4 8%

Since April 2010, Google Webmaster Tools shows the percentage of clicks for every page, depending on its position, which evolves over time and compares it with the number of displays.
The information it provides is more useful than the statistics above.