Would you mind fleshing that out? Just posting two words and linking to wikipedia doesn't add much to the conversation, and people who care about maintaining the level of discourse on HN might heavily downvote you.
Suppose that we want to study the underlying genetics of a disease. To do this, we want to look at some people with the disease (cases) and some people without (controls). The trouble is that if the disease makes people more likely to die, they won't be enrolled in your study.
This is of particular concern for fast moving diseases like pancreatic cancer. In general this effect manifests as a bias against people with more severe disease -- occasionally these are exactly the people you want to study.
The simplest way to get around this is with an alternative study design. Rather than ascertaining cases and controls, you enroll a large number of individuals in a cohort. After that, you sit back and track the cohort for decades and watch for disease to emerge.
A famous example of this is the Religious Orders Study (http://www.rush.edu/rumc/page-1099611542043.html). A large number of Catholic brothers and sisters graciously agreed to participate on a study of Alzheimer's disease. The researchers recruited a cohort of non-demented clergy and have been tracking them for years. They perform annual cognitive tests on the participants to assess mental decline. All participants were gracious enough to consent to post-mortem brain donation. It would be impossible to get this sort of data with a case/control study.
Another type of ascertainment bias is population stratification which can generate all sorts of misleading results. Imagine that you're a scientist in Boston and you want to study sickle cell anemia. You phone up a doctor friend and say "Please send me the next 10 sickle cell cases and the next 10 non-sickle cell cases." After spending $300,000 and a year and a half on the project you find some great mutations. Ten minutes later you notice that all the markers you found are strongly associated with being African-American. Your cases included 9 persons of African ancestry and one of Mediterranean ancestry, while your controls matched the particular demographic blend that you'd expect to find around Boston. Oops.
I hope that helps.
p.s.
Is anyone in London looking for smart people? E-mail me. ;)
Similar to the famous Boston cemetery study. It shows the age at death of people in a cemetery drops linear as you approach the current date. whereas people 100years ago all died in their 60-80s
this bias tends to come up frequently on HN (often as a reminder that this community shares more articles about businesses that succeed than those that fail), so many may recognize it (and upvote it) --- it's sort of like saying tl;dr and summarizing the article it it's own way.
I think there's value in pointing out a specific topic that might otherwise be difficult to discover. Sure, it would be even better to summarize it a bit, but it's still valuable enough.
