Recent years there has been an increase in the number of shark attacks along the Florida beaches. Tahir has been hired by the state to investigate this phenomenon. He wonders what change has prompted more shark attacks. He consults experts at Marine Science Center in New Smyrna Beach, shark capital of the world. The data over the past 8 years show a steady increase in population and an increase in people who use the water for recreation. Tahir believes that the steady increase in beach goers is related to the increase in shark attacks.

What is the independent variable in Tahir's lab?

Question

Recent years there has been an increase in the number of shark attacks along the Florida beaches. Tahir has been hired by the state to investigate this phenomenon. He wonders what change has prompted more shark attacks. He consults experts at Marine Science Center in New Smyrna Beach, shark capital of the world. The data over the past 8 years show a steady increase in population and an increase in people who use the water for recreation. Tahir believes that the steady increase in beach goers is related to the increase in shark attacks. 

What is the independent variable in Tahir's lab?