Sarah wants to know which brand of nail polish lasts the longest without chipping.She buys 4 types of nail polish - Essie (which she normally uses,) butter, OPI, and Sally Hanse. Every Sunday for 4 weeks she paints her nails with a different brand of nail polish and records how many days she lasts before she gets her first chip. She makes sure to use the same bottom coat and top coat with each types of nail polish, and she makes sure to do the same weekly routine each week so that her nails aren't getting treated more roughly
different weeks. What is the independent variable