In statistics<unk>  refers to the absence of registered<unk>  for a given variable.<unk>  is frequent in clinical research.  It is an important source of bias, reducing the consistency (precision or reproducibility) of the study. It can have an important effect on the conclusion of the study potentially leading to invalid results drawn from the<unk> .
Missing data
Biased sample
Sample size
Absolute deviation
Block design
Attrition 
Cumulative incidence
Degrees of freedom 