The phrase "selection bias" most often refers to the distortion of a statistical analysis, resulting from the method of collecting samples.
[13] Lost to follow-up, is another form of Attrition bias, mainly occurring in medicinal studies over a lengthy time period.
Non-Response or Retention bias can be influenced by a number of both tangible and intangible factors, such as; wealth, education, altruism, initial understanding of the study and its requirements.
[17] Astronomical existential risks might similarly be underestimated due to selection bias, and an anthropic correction has to be introduced.
More generally speaking volunteer response can be put down to individual altruism, a desire for approval, personal relation to the study topic and other reasons.
An assessment of the degree of selection bias can be made by examining correlations between exogenous (background) variables and a treatment indicator.