The premise is that we can't apply regular statistical approaches that rely on specific sampling techniques - if we're not using those sampling techniques. And in healthcare projects we seldom use scientific sampling methods, instead using a non-probability, judgement sampling method. In their paper they describe the method and explain the impact for healthcare settings.
The paper is in a subscription only journal. (Bah-humbug...) So if you're not able to access this journal, you can learn more about judgement sampling at these places:
- FULL ACCESS PAPER: Sampling for qualitative research, by Martin Marshall
- List of definitions of various sampling methods
- What is judgement sampling?
- When to use and setback of judgement sampling
- Advantages & disadvantages of judgement sampling
The Perla & Provost paper can be found here:
Qual Manag Health Care. 2012 Jul;21(3):169-75.
Judgment sampling: a health care improvement perspective.