Corrigendum to: comparing the use of direct observation, standardized patients and exit interviews in low- and middle-income countries: a systematic review of methods of assessing quality of primary care. Issue 6 (14th May 2021) - Aujla, Navneet; Chen, Yen-Fu; Samarakoon, Yasara; Wilson, Anna; Grolmusova, Natalia; Ayorinde, Abimbola; Hofer, Timothy P; Griffiths, Frances; Brown, Celia; Gill, Paramjit; Mallen, Christian; Sartori, J o; et al.