Which of the following is not an assumption of the multiple linear regression model?

The level of measurement of the data for the dependent variable is at least ordinal.

The error terms are independent.

At any combination of values ofx₁,x₂, . . . ,x_k, the population of potential error term values has a mean equal to zero.

The population of error terms has a normal distribution.

Populations of error terms observed at different combinations of values of the independent variable (x₁, x₂,. . .,x_k) have equal variances.