The correct completion of the sentence is:

"The mean squared error (MSE) is measured in units of the Y variable. The lower the MSE, the higher the R-squared."

So the correct option is: in units, lower, higher.

Explanation:

• The MSE is measured in the same units as the dependent variable (Y).
• R-squared measures the proportion of the variance in the dependent variable that is predictable from the independent variables. A lower MSE generally means a better fit of the model, and consequently, a higher R-squared.

Would you like further clarification or examples on MSE or R-squared? Here are some related questions:

1. How does MSE relate to model accuracy?
2. What does R-squared signify in regression models?
3. How do we interpret the relationship between MSE and model fit?
4. What happens to R-squared when MSE is high?
5. Can you explain the difference between MSE and RMSE?

Tip: To improve model performance, always check the MSE and consider optimizing your model to reduce this error.