Editing Randomized controlled trial (section)

=== By study design ===
One way to classify RCTs is by [[study design]].  From most to least common in the healthcare literature, the major categories of RCT study designs are:<ref name="Hopewell-2010">{{Cite journal |vauthors=Hopewell S, Dutton S, Yu LM, Chan AW, Altman DG |date=March 2010 |title=The quality of reports of randomised trials in 2000 and 2006: comparative study of articles indexed in PubMed |journal=BMJ |volume=340 |pages=c723 |doi=10.1136/bmj.c723 |pmc=2844941 |pmid=20332510}}</ref>
* [[Parallel study|Parallel-group]] – each participant is randomly assigned to a group, and all the participants in the group receive (or do not receive) an intervention.<ref>{{Cite journal |vauthors=Kaiser J, Niesen W, Probst P, Bruckner T, Doerr-Harim C, Strobel O, Knebel P, Diener MK, Mihaljevic AL, Büchler MW, Hackert T |date=June 2019 |title=Abdominal drainage versus no drainage after distal pancreatectomy: study protocol for a randomized controlled trial |journal=Trials |volume=20 |issue=1 |page=332 |doi=10.1186/s13063-019-3442-0 |pmc=6555976 |pmid=31174583 |doi-access=free}}</ref><ref>{{Cite journal |vauthors=Farag SM, Mohammed MO, El-Sobky TA, ElKadery NA, ElZohiery AK |date=March 2020 |title=Botulinum Toxin A Injection in Treatment of Upper Limb Spasticity in Children with Cerebral Palsy: A Systematic Review of Randomized Controlled Trials |journal=JBJS Reviews |volume=8 |issue=3 |pages=e0119 |doi=10.2106/JBJS.RVW.19.00119 |pmc=7161716 |pmid=32224633 |doi-access=free}}</ref>
* [[Crossover study|Crossover]] – over time, each participant receives (or does not receive) an intervention in a random sequence.<ref>{{Cite book |title=Design and Analysis of Cross-Over Trials |vauthors=Jones B, Kenward MG |publisher=London: Chapman and Hall |year=2003 |edition=Second}}</ref><ref>{{Cite book |title=Linear and Nonlinear Models for the Analysis of Repeated Measurements |vauthors=Vonesh EF, Chinchilli VG |publisher=London: Chapman and Hall |year=1997 |pages=111–202 |chapter=Crossover Experiments}}</ref>
* [[Cluster randomised controlled trial|Cluster]] – pre-existing groups of participants (e.g., villages, schools) are randomly selected to receive (or not receive) an intervention.<ref>{{Cite journal |vauthors=Gall S, Adams L, Joubert N, Ludyga S, Müller I, Nqweniso S, Pühse U, du Randt R, Seelig H, Smith D, Steinmann P, Utzinger J, Walter C, Gerber M |date=8 November 2018 |title=Effect of a 20-week physical activity intervention on selective attention and academic performance in children living in disadvantaged neighborhoods: A cluster randomized control trial |journal=PLOS ONE |volume=13 |issue=11 |pages=e0206908 |bibcode=2018PLoSO..1306908G |doi=10.1371/journal.pone.0206908 |pmc=6224098 |pmid=30408073 |doi-access=free}}</ref><ref>{{Cite journal |vauthors=Gladstone MJ, Chandna J, Kandawasvika G, Ntozini R, Majo FD, Tavengwa NV, Mbuya MN, Mangwadu GT, Chigumira A, Chasokela CM, Moulton LH, Stoltzfus RJ, Humphrey JH, Prendergast AJ |date=March 2019 |title=Independent and combined effects of improved water, sanitation, and hygiene (WASH) and improved complementary feeding on early neurodevelopment among children born to HIV-negative mothers in rural Zimbabwe: Substudy of a cluster-randomized trial |journal=PLOS Medicine |volume=16 |issue=3 |pages=e1002766 |doi=10.1371/journal.pmed.1002766 |pmc=6428259 |pmid=30897095 |doi-access=free}}</ref>
* [[Factorial experiment|Factorial]] – each participant is randomly assigned to a group that receives a particular combination of interventions or non-interventions (e.g., group 1 receives vitamin X and vitamin Y, group 2 receives vitamin X and placebo Y, group 3 receives placebo X and vitamin Y, and group 4 receives placebo X and placebo Y).
An analysis of the 616 RCTs indexed in [[PubMed]] during December 2006 found that 78% were parallel-group trials, 16% were crossover, 2% were split-body, 2% were cluster, and 2% were factorial.<ref name="Hopewell-2010" />