Epidemiological studies investigate the causes of particular health outcomes based on associations with different risk factors that are recorded in the study. The risk factors are often selected a priori. Whilst a given study will investigate particular exposures, the risk of developing the health outcome can be affected by other factors. In some cases, these factors may ultimately be responsible for some or all of the deduced relationship between health outcomes and exposure variables of interest. This distortion leads to an invalid comparison and the distorting factors are called confounding factors or confounding variables. As an example, with studies that investigate the relationship between increasing daily air temperature (the exposure variable) and heat-related mortality (the outcome variables), confounding factors that are often included are daily air pollution and relative humidity (Baccini et al. 2011; Ma et al. 2011; Porta 2008).