Parallel and crossover studies are conducted to confirm the efficacy of any drug. Parallel studies are conducted when more accurate results are desired but it is expensive while crossover studies are conducted when you have less budget. In parallel study 2 groups are taken. One group is administered with the drug and other with placebo (control group) while in the crossover study same group is given with drug as well as with placebo. There is no control group in the crossover study.