New drugs being developed can be "easy" drugs or "difficult" drugs.

In order to know whether your drug candidate is safe and effective, you're going to test it in a series of clinical trials. In each trial, you'll recruit some number of patients, give each patient the treatment, placebo, or a comparator drug, wait some time, and test them for pre-specified endpoints.

Within that framework, however the trials for different drugs will differ greatly. (And the different phases of a single drug's trials may differ by even more!) Typically, the greatest axes of variation will be:

• Who are your patients?
  • How common is the indication that you're treating? How often do people go to your trial site to get treatment for it? How many of them want to be in a trial?
  • Is your trial taking anyone with the disease? / Is it only for people who are not responding to some other standard treatment?
  • Are your patients otherwise healthy? / Do they have elevated risks for other complications?
• What condition does the drug affect?
  • Are you trying to change something that patients already have? / Are you trying to stop them from developing something else?
  • If you're preventing something, what fraction of your patients will develop it without treatment?
  • What change in the condition are you trying to measure? Is it yes-or-no or on a scale?
  • If the drug "works", what fraction of cases will it change enough for you to measure?
• Where does the trial take place?
  • Are you treating patients in a hospital?