van Winden 1998.

Study characteristics
Methods	RCT. Cross‐over design (1‐week washout period). Location: single centre in the Netherlands. Duration: 6 weeks (each treatment 2 weeks and 1 week wash in/wash out period).
Participants	22 participants with CF confirmed by sweat test or DNA mutation analysis. Mean age 12 years; range 7 ‐ 17 years. Sex: 12 males, 10 females. Clinically stable for 2 weeks before study.
Interventions	Flutter versus PEP mask. 2x daily, 2 weeks in each arm, 1 week wash‐in and wash‐out period.
Outcomes	FVC, FEV1, RV/TLC, FEF25-75% predicted, participant satisfaction. Outcomes were all measured before and after each treatment intervention.
Notes	Outcome assessor blinded.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Described as randomised, but method not discussed.
Allocation concealment (selection bias)	Unclear risk	Not discussed.
Blinding (performance bias and detection bias) All outcomes	Low risk	Participants and clinicians could not be blinded, but outcome assessor was blinded.
Incomplete outcome data (attrition bias) All outcomes	Low risk	All participants completed the study and their data were included.
Selective reporting (reporting bias)	Unclear risk	Not clear what happened in the run‐in or wash out period between cross‐over.
Other bias	Unclear risk	None identified.