when reading this, I don't know why but a question suddenly came into my mind, why do we need so complicated/fancy models in social science research, specifically, except core independent variables and outcome variables, why do we use covariate/ control variables in a given model. I had an insight from a professor's explanation: for natural science, most objects of study are homogeneous and scientists can have a good control of interference in lab environment with careful experimental design. However, in terms of social science phenomenons, they are so complicated and are impacted by so many factors, including which we already know, and also a lot of which we don't know yet, let alone the subjects of social science study are so unique and heterogeneous. So we have to use advanced model to get closer to understanding those complex phenomenons, and we have to try our best to control the covariates we already know to carefully test the real relationship between independent variables and dependent variable. In addition, because we can not know or measure all factors that will impact a certain complex phenomenon, this is one of the reasons that a model is a simplification or approximation of reality and hence will not reflect all of reality.