Dooms
“Wikipedia: The healthy user bias i… is a sampling bias: the kind of subjects that voluntarily enroll in a clinical trial and actually follow the experimental regimen are not representative of the general population. They can be expected, on average, to be healthier”
Yes! you nalied it
Now before we go on to the MMR study which you seem to have never never even glanced at, lets make sure this point is clear. This is a simple point and as APY helpfully pointed out we are 1300 posts in. Focus on this point with a little effort we can do it!
Try not to get distracted with irrelevant side distractions like confusing caring “More education” with caring about health, or “organic” with being healthy. These are nonsense and are distractions.
Set them aside and focus on healthy user bias, as you correctly said it is a “sampling error”
What this means (again from wikipedia”) “sampling error is incurred when the statistical characteristics of a population are estimated from a subset, or sample, of that population. Since the sample does not include all members of the population, statistics on the sample, such as means and quantiles, generally differ from the characteristics of the entire population, which are known as parameters. For example, if one measures the height of a thousand individuals from a country of one million, the average height of the thousand is typically not the same as the average height of all one million people in the country. Since sampling is typically done to determine the characteristics of a whole population, the difference between the sample and population values is considered an error”
Or another example if I recruit for my study using the internet, then the sample that I study would have this sample bias, I am automatically excluding those who may not have the internet.
With me so far?
Now lets look at the specific sampling error known as “healthy user bias”
If I look at whether Prostate cancer screenings save lives, A possible flaw is the healthy user bias, those who go to doctors to get screened are more likely to care about their health and live longer
Or with vaccines (from some anti-vaccine website:)
“Most vaccine safety studies are observational, and accordingly, do not include researcher control of vaccine exposure. …. A big problem is that vaccinated and unvaccinated people are not matched. Critical differences include:
1) Healthy people are more likely to choose to be vaccinated. People with chronic diseases or health issues tend to avoid the risk of vaccination.
2) People that choose vaccination tend to have other “health seeking” behaviors, such as having a better diet and exercising, or getting regular screenings and medical tests.
These differences create “healthy user bias” (HUB) or the “healthy user effect” in vaccine studies”
Lets now look at the MMR study,
Are you with me so far?
No changing of definitions later, can we agree to “lock in” this definition.