My assumption is that those with higher target have a higher spread of results i.e. their level of anticoagulation changes more dramatically (from their target) than those with lower targets.

What I would effectively be doing is looking at the variation of theirs results over time (which I am assuming to be of normal distribution) and comparing the spread of these results between the 3 groups. Can anybody point me in the right direction regarding an appropriate statistical test anything else I might be overlooking? Thanks, Dan