Fill in the blank. If there is __________ between \(x_{1}\) and \(x_{2}\) in a least-squares regression model, we would build a model of the form \(\hat{y}_{i}=\beta_{0}+\beta_{1} x_{1 i}+\beta_{2} x_{2 i}+\beta_{3} x_{1 i} x_{2 i}+\varepsilon_{i}\).