Tanzania's growth recovery was a statistical artifact

Tanzania's GDP series until the 1990s assumed proportionality—that when the recorded economy declined the unrecorded economy declined too—but when the World Bank rebased the series in the 1990s it adopted the opposite assumption that the informal economy grew when the formal economy declined; this methodological switch settled a deep academic question at the statistical office and produced a sudden apparent growth recovery, implying the 1980s decline was overestimated and the 1990s liberalization recovery was overestimated.