A study is conducted to determine what factors drive used car prices. The study initially examined two variables: the make of the car and the number of miles driven on the car. Not surprisingly, the group commissioning the study found that there was a strong relationship between these two variables and price. In an attempt to explain more of the variation in price the researcher added a thirdvariable:the car's age (in years).A prior study had shown that age was a significant factor in explaining used car selling prices. When added to the new model, however, the group found that although adding age explained more of the variation in price, age was no longer significant when combined with the car's make and miles. This problem is an example of: