The following example shows the importance of considering “ecological fallacy” to interpretate any data results. (From Wikipedia)
In the United States presidential elections of 2000, 2004, and 2008, wealthier states (states with higher per capita incomes) tended to vote Democratic and poorer states tended to vote Republican. Yet wealthier voters tended to vote Republican and poorer voters tended to vote Democratic. For example, in 2004, the Republican candidate, George W. Bush, won the fifteen poorest states, and the Democratic candidate, John Kerry, won 9 of the 11 wealthiest states. Yet 62% of voters with annual incomes over $200,000 voted for Bush, but only 36% of voters with annual incomes of $15,000 or less voted for Bush.