I'm going over the DB you attached and it looks like there were a few things I forgot about when coming up with my recommended structure: Namely employee departments and qualifications.
Are you interested in tracking what department an employee worked in for each period?
How exactly do the qualifications work? Is it a simple tiered system where if you have "Level 2", you automatically have "Level 1" as well? Or is it possible, for example, to have "Level 2" and "Level 3" qualifications but not "Level 1"?
And are you interested in tracking the employee qualifications for each period?