I hope this is the appropriate forum for this:
I would be very grateful if anyone were able to advise on this; my background in statistics is relatively limited. I’m trying to compare the use of different antibiotics on a ward either side of a change in policy which should have discouraged the use of A/B/C and potentially increased the use of D/E/F.
The problem I have is that instead of usage data, which is very difficult to collect, I’ve got supply data for each month of a two year period (one year either side of the policy change). What I’ve noticed is that when you simply plot antibiotic supply against month, the result is almost a sine wave! This makes me think that the data points are not entirely independent due to a scenario something like this:
January – Lots of antibiotic A were used
February – cupboard appeared bare so lots of antibiotic A were ordered
March – Quite a lot of antibiotic A left in cupboard, less ordered
April – etc. etc.
I’m not at all sure what I’m “allowed to do” with the data? I've applied non-parametric testing and demonstrated significance but am concerned that that is not appropriate.
Any help gratefully received; obviously significant help will be credited at publication.