Eastwood, B., N. Jepsen, K. Coulter, C. Wong, and I. Zeng. “Challenges of Undertaking a Clinical Trial Using Bubble-PEP in an Acute Exacerbation of Chronic Obstructive Pulmonary Disease: A Feasibility Study”. New Zealand Journal of Physiotherapy, vol. 44, no. 1, Mar. 2016, pp. 8–16, doi:10.15619/NZJP/NZJP/44.1.02.