Efficient in / efficient at
Which one is correct? Why?

a) "My goal is to provide evidence about their lack of efficiency AT studying or practising."

b) "My goal is to provide evidence about their lack of efficiency IN studying or practising."

Note: "They" refers to "the students".

Question

Efficient in / efficient at
Which one is correct? Why?

a) "My goal is to provide evidence about their lack of efficiency AT studying or practising."

b) "My goal is to provide evidence about their lack of efficiency IN studying or practising."

Note: "They" refers to "the students".

Answer

My opinion is that both are correct.  The choice of preposition is often arbitrary.  I would choose "at".  Somebody else might choose "in".  I think that "for" or "with" and probably some others could be used.