How do open and private test problems differ?
The difference is in access and novelty, not difficulty. Open problems are open to access without registration and payment. They are meant for entertainment and edification. Individual open problems may be discussed in detail on our blog. When a test is assigned to a candidate, one private problem is chosen randomly for the test. While they are new, private problems can be used effectively to test candidates. When they cross a certain age, they will be retired into the open pool and new private problems introduced into the private pool.
