In April, Nigeria revised its books. Overnight, GDP nearly doubled, and Nigeria shot past South Africa to become the continent's largest economy.

This wasn't gimmickry or dodgy accounting. According to Nigerian officials, it was a correction—one that's likely to be repeated in developing countries around the world. Just last month, Kenya recalculated its GDP upward, to the tune of 25 percent.

The episodes highlight what those in the development world have known for decades and mostly ignored: Africa has a data problem. A big data problem. Official GDP figures for most African countries, even relative powerhouses like Angola, are dramatically understated—literally off base, for they're measured against baselines so old they're obsolete. Nigeria was calculating its income against a baseline