Liberia's score showed the biggest increase, from 32 in 2004/05 to 44 in 2008/09, the latest years for which data are available. Angola's score also rose steadily over the four years, from 31 to 39, while Togo's improved from 36 to 43.

All three countries nevertheless remained in the bottom half of the continent's rankings. Of 53 nations surveyed, Liberia was in 36th place, just ahead of Niger and Mauritania and immediately behind Cameroon and Ethiopia.

Togo was in 39th and Angola in 43rd place. Eritrea's score on the index dropped from 40 to 33 over the four years, and it was ranked in 49th place, only four places above bottom-ranked Somalia, where the federal government does not even control the whole of the capital, Mogadishu.

Madagascar, where there was an unconstitutional seizure of power 18 months ago, saw its score on the index drop from 56 to 48, but it remained higher on the rankings, as the 29th best-governed country in Africa.

The survey indicates that the continent's best-governed region is Southern Africa, with an average score on the index of 57, followed by North Africa (54), West Africa (50), East Africa (45) and Central Africa (38).

The Ibrahim index is produced by the Mo Ibrahim Foundation, which was founded by the Sudanese cellphone entrepreneur of the same name. The foundation describes the index as "a tool to hold governments to account and frame the debate about how we are governed."

Citing what it saw as the most interesting trends seen in this year's survey, the foundation said more than 40 countries had seen "some form of improvement" in the categories of sustainable economic opportunity and human development.

"Generally, African citizens are healthier and have more access to economic opportunities than was the case five years ago …

"However, the category that gender sits within, participation and human rights, makes for less encouraging reading. Thirty of Africa's 53 states have declined in participation and human rights performance over the past five years - notwithstanding some improvements around gender issues.

Scores achieved in selected countries:
Botswana - 74
South Africa - 70
Namibia - 67
Ghana - 64
Lesotho – 60
Egypt – 59
Senegal – 57
Tanzania – 54
Malawi and Mozambique – 52
Uganda and Libya – 51
Kenya – 50
Rwanda and Madagascar – 48
Ethiopia and Liberia – 44
Nigeria - 43
Cote d'Ivoire – 38
Guinea and Equatorial Guinea – 36
Eritrea and Sudan - 33
Zimbabwe and Democratic Republic of Congo – 32