World oil price is fixed by the major oil producers and relevant oil producer organization such as the OPEC.

To study the impact on the economy of oil price you should selected different types of countries:

· Main oil producers and exporters;

· Main oil consumers and importers;

· Middle size importers countries; and

· Small size importers countries.

Jorge Morales Pedraza gave the best answer you could have.

That is because the impact of oil price will tend to have positive correlation if you are observing an important oil producer (like Brazil, Russia, Saudi Arabia, Venezuela, etc). But oil price will tend to have negative impact for countries that import oil to sustain their industrial production (Germany, China, USA, etc.).

You definitely can't treat those countries as the same group.

Thanks #jorge #apoena for your help