What is biostatistical  application and their objective?

I need help with part b please to find the two models which use the best subset of variables. thanks

1. What is the independent variable? How is it operationally defined? 2. What type of variable is the independent variable (situational, response, participant, or mediating)? 3. What is the dependent variable? How is it operationally defined? 4. What type of variable is the dependent variable (situational, response, participant, or mediating)? 5. Is it a nonexperimental or experimental design? Why?   For each of the kinds of validity below, what concerns about that type of validity do you have with this study? If you don’t have any concerns about that kind of validity, then explain why you think the study does a good job addressing it. If you would need additional information to evaluate that kind of validity, specify what you would need to know. A. Construct validity B. Internal validity C. External validity D. Statistical conclusion validity

Overhead Door (OD) Corporation’s founder, C. G. Johnson, invented the upward-lifting garage door in 1921 and the electric garage door opener in 1926. Since then OD has been a leading supplier of commercial, industrial, and residential garage doors sold through a nationwide network of more than 450 authorized distributors. They have built a solid reputation as a premier door supplier, commanding 15 % share of the market. Suppose that customers assess door quality first in terms of the ease of operation, followed by its durability. The quality improvement team (QIT) might then assign an engineering team to determine the factors that contribute to these two main problems. Smooth operation of a garage door is a critical quality characteristic that affects both problems: If a door is too heavy, it’s difficult and unsafe to balance and operate; if it’s too light, it tends to buckle and break down frequently or may not close properly. Suppose the design engineers determine that a standard…