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218 7. INTERACTIONSlog(rgdppc_2000)6 7 8 9Africalog(rgdppc_2000)7 8 9 10 11not Africa0 1 2 3 4 5 6rugged0 1 2 3 4 5ruggedFIGURE 7.2. Separate linear regressions inside and outside of Africa, for log-GDP against terrain ruggedness. e slope is positive inside Africa, butnegative outside. How can we recover this reversal of the slope, using thecombined data?# extract countries with GDP datadd

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