Physical Therapy and Practice Variation

As I was finishing a sprint triathlon this weekend, instead of observing the beauty of fall on Michigan State's campus, my mind was wandering and thinking.  One question I mulled about was that of practice variation. 

Why is there practice variation?    What data could be collected to know the extent of the variation and factors that have the largest influence on practice variation?   What factor(s) could be changed to have the largest impact on reducing practice variation? 

In looking at the big picture, it appears to me there could be multiple reasons for practice variation.  Yes, I believe physical therapists inherently are responsible for their practice and clinical decision-making, but at the same time, multiple factors influence physical therapists.... the APTA, state regulations, academia, research, third party payors, and departmental policy and procedure.  Even patients themselves may play a role in practice variation.

As we are focusing on becoming the experts in neuromusculoskeletal care, how can we overcome this obstacle?  At the same time, there is always variation in processes, so what is an acceptable level of variation?

Selena