What evidence can you find that has scientifically looked at the issue of intubation currency and competency?

What evidence can you find that has scientifically looked at the issue of intubation currency and competency? How many tubes and in what circumstances do you need to place to obtain competency in the first place? How often do you need to intubate to maintain currency? There should be some good objective data out there. https://www.youtube.com/watch?v=atueYCUU_Nw&feature=youtu.be