This threat to external validity occurs when a treatment effect ...

[elizabeth18]

This threat to external validity occurs when a treatment effect is found for only a specific set of
  participants.
 
  A) reactive arrangements B) interaction of selection and treatment
  C) multiple-treatment interference D) interaction of testing and treatment

Question 2

Why can we not estimate population parameters like the mean or standard deviation when
  using nonparametric tests?
 
  A) When you violate assumptions of normality, means and SDs are inaccurate.
  B) because you cannot calculate means and SDs from nominal or ordinal data
  C) because these have no meaning in nonparametric analyses
  D) Actually, you can estimate the mean and SD with nonparametric tests.

[bd5255]

Answer to Question 1

B

Answer to Question 2

B

[elizabeth18]

Thanks for the timely response, appreciate it

[bitingbit]

Gracias!