You want to know whether a new juice diet introduced this year helps people sleep well. You have observational data on sleep and eating habits and other personal characteristics. What regression has the greatest internal validity?
A. Sleep = B0_hat + B1_hat*juicediet + u_hat
B. Sleep = B0_hat + B1_hatjuicediet + B2_hatsleeplastyear + u_hat
C. Sleep = B0_hat + B1_hatjuicediet + B2_hatlastdigitofphonenumber+ u_hat
D. Juice Diet= B0_hat + B1_hat*sleeplastyear+ u_hat
Answer: B. Sleep = B0_hat + B1_hatjuicediet + B2_hatsleeplastyear + u_hat