What's household bias in a survey?

In the social sciences, researchers often use surveys to
try to find out about the incidence or causes of some social phenomenon.  For example,
they might use a survey to find out about how common drug use is among various segments
(for example, people of different ages or income levels) of the population.  Since they
cannot survey every member of that segment (for example, all 15 year olds in the
country) they survey only a sample and base conclusions about the whole population on
the responses elicited from the smaller sample.

When doing
this, it is very important to avoid bias in the sample.  Household bias is one possible
sort of bias that can skew a sample.  Household bias occurs when a researcher samples
the same number of people from different groups even though those groups are of
different sizes.  For example, if you wonder about drug use at a high school and you
poll an equal number of athletes and non-athletes, you will likely get household bias
because there are more non-athletes than athletes in the whole student body.  To avoid
this, it is important to poll a given type of person in proportion to their presence in
the overall population.  If the school is 70% non-athlete and 30% athlete, you need to
have your sample consist of 70% non-athletes and 30% athletes.