It makes it really hard to work out what's happening, especially if you want the result to match existing standards.

For a real world example of this see deworming schoolchildren.

People looking at the educational effects of deworming children reach different conclusions because some of them use a medical model and some of them use an economics model.

http://www.cochrane.org/news/educational-benefits-deworming-...

http://www.cochrane.org/CD000371/INFECTN_deworming-school-ch...

Talked about in this More or Less episode:

http://www.bbc.co.uk/programmes/b0659q1f

http://www.theguardian.com/society/2015/jul/23/research-glob...