Methley, A. M., Campbell, S., Chew-Graham, C., McNally, R., & Cheraghi-Sohi, S. (2014). PICO, PICOS and SPIDER: A comparison study of specificity and sensitivity in three search tools for qualitative systematic reviews. BMC Health Services Research, 14(1), 579.
Additional letters (for PICOT, PICOS, PICOTS, PICOTTS)
Haynes, R. B., Richardson, W. S., Rosenberg, W., & Sackett, D. L. (1997). Evidence based medicine: How to practice and teach EBM. New York: Churchill Livingstone.
Cooke, A., Smith, D., & Booth, A. (2012). Beyond PICO: the SPIDER tool for qualitative evidence synthesis. Qualitative health research, 22(10), 1435–1443. https://doi.org/10.1177/1049732312452938.
Moola, S., Munn, Z., Sears, K., Sfetcu, R., Currie, M., Lisy, K., Tufanaru, C., Qureshi, R., Mattis, P., Mu, P. & (2015). Conducting systematic reviews of association (etiology). International Journal of Evidence-Based Healthcare, 13 (3), 163-169. https://doi.org/10.1097/XEB.169.0000000000000064.
Booth, A. (2006). Clear and present questions formulating questions for evidence based practice. Library Hi Tech, 24(3), 355–368.
Booth, A., Noyes, J., Flemming, K., Moore, G., Tunçalp, Ö., & Shakibazadeh, E. (2019). Formulating questions to explore complex interventions within qualitative evidence synthesis. BMJ global health, 4(Suppl 1), e001107. https://doi.org/10.1136/bmjgh-2018-001107.