Cancer screening is the effort to detect cancer early, during its pre-clinical phase—the time period that begins with an abnormal cell and ends when the patient notices symptoms from the cancer. It has long been known that some people have cancers with short pre-clinical phases (fast-growing, aggressive cancers), while others have cancers with long pre-clinical phases (slow-growing cancers). And this heterogeneity has an unfortunate implication: namely, screening tends to disproportionately detect slow-growing cancers (because they are accessible to be detected for a long period of time) and disproportionately miss the fast-growing cancers (because they are only accessible to be detected for a short period of time)—the very cancers we would most like to catch. For more information, see Screening (medicine)#Length time bias.