Two primary types of field studies have been used to investigate the association of ventilation rates with health and perception outcomes. In cross-sectional studies, a type of observational study, data on health (or perception) outcomes, ventilation rates, and other relevant factors are collected in multiple buildings or building spaces and analyzed with statistical models to determine the strength and uncertainty in the associations of ventilation rates with the health outcomes. A major weakness of this study design is that many factors other than ventilation rate which vary among the buildings may influence the health outcomes, confounding the association of ventilation rate with the health outcome. Confounding occurs when a factor related to the outcome is distributed differently in the groups being compared and distorts the relationship being studied. Another potential weakness is that occupants with substantial adverse health effects from an exposure may preferentially leave the buildings or be absent more often. The better cross-sectional studies control for potential confounding factors in the study design or data analyses. Cross-sectional studies can find statistical associations but, without other supporting findings, such studies can not confirm cause and effect relationships.