a variable, usually unobserved, that influences the association between the variables of primary interest
confounding
occurs when two explanatory variables are associated with a response variable, but are also associated with each other
observational study
the values for the response variable and explanatory variables are observed for the sampled subjects, without anything being done to them
experimental study
an experiment is conducted by assigning subjects to certain experimental conditions (treatments) and then observing the outcome on the response variable
simple random sample of n subjects from a sampling frame
each possible sample of size n has the same chance of being selected
sampling bias
the sample is not random or the sampling frame does not represent the full population (undercoverage)
non-sampling bias
bias that does not occur as a result of the sampling design
nonresponse bias
a form of non-sampling bias in which some sampled subjects cannot be reached or refuse toparticipate
response bias
the participant is not honest when answering questions or answers questions wrongly; misleading questions
