[Exploring the abnormal correlation between obesity and higher survival rate based on directed acyclic graphs and collider bias theory].

In the general population, obesity (high BMI) is often considered as an important risk factor for many diseases and premature death. However, in patients with a certain disease, the risk of death in people with obesity was lower than that in people with normal weight. This abnormal correlation has caused widespread discussion, and different explanations have been given by physiology and epidemiology. Based on causal inference theory, this study used directed acyclic graphs to introduce the collision bias (also called collision-stratification bias). Through example demonstrations, this study explored whether and why the abnormal correlation between obesity and higher survival rates in patients with hyperglycemia exists. Finally, it was concluded that this abnormal correlation among patients with a certain disease (e.g., hyperglycemia in the current analysis) was partly or even totally caused by collider bias.