 There are four phases at stages of clinical trials. Phase one is for safety and acceptability and it is done among healthy volunteers. Phase two is for dosing and scheduling that is the efficacy of the drug and also for long-term safety. This is among low-risk patients and phase three is for effectiveness and this is among high-risk patients. Phase four is the surveillance after the drug has been marketed. So this is post-marketing surveillance. It is from the community-based data. As we go up in the phase of the clinical trial, the number of participants increases that is up to 50 in phase one, 100 to 500 in phase two, around 1000 in phase three and more than 1000 in phase four. So you need to remember objectives, sample size and participant of each trials that is very important in case of phases or stages of clinical trials.