Francis , J J , Johnston , M , Robertson , C , Glidewell , L , Entwistle , V A , Eccles , M P & Grimshaw , J M 2010 , ' What is an adequate sample size? Operationalising data saturation for theory-based interview studies ' Psychology & Health , vol. 25 , no. 10 , pp. 1229-1245 . DOI: 10.1080/08870440903194015