Cancer Screening Overview (PDQ®): Screening - Patient Information [NCI] - How do Screening Tests Become Standard Tests?
Case-control studies are like cohort studies but are done in a shorter time. They do not include many years of follow-up. Instead of looking forward in time, they look backward. In case-control studies, information is collected from cases (people who already have a certain disease) and compared with information collected from controls (people who do not have the disease). For example, a group of patients with melanoma and a group without melanoma might be asked about how they check their skin for abnormal growths and how often they check it. Based on the different answers from the two groups, the study may show that checking your skin is a useful screening test to decrease the number of melanoma cases and deaths from melanoma.
Evidence from case-control studies is not as strong as evidence from clinical trials or cohort studies.
Ecologic studies report information collected on entire groups of people, such as people in one city or county. Information is reported about the whole group, not about any single person in the group. These studies may give some evidence about whether a screening test is useful.
The evidence from ecologic studies is not as strong as evidence from clinical trials or other types of research studies.
Expert opinions can be based on the experiences of doctors or reports of expert committees or panels. Expert opinions do not give strong evidence about the usefulness of screening tests.
New ways to screen for cancer are being studied in clinical trials.
Clinical trials are taking place in many parts of the country. Check the NCI Web site for NCI's list of cancer screening trials that are now accepting patients.