I have a dataset with several variables. The dependent variable is income, and the most important independent is age. I wish to model the relation between income and age, while taking into account other covariates. I have two questions: 1. How do I choose the right nonlinear model? I ask about nonlinear for two reasons: Theoretically, the relation should be quadratic. In my data, it looks exponential (image attached). 2. The sample is large, so small P-Values are likely. How do I evaluate the correctness of the model? In a t-test, I can always calculate the effect size. What is the effect size here? Should I use predictions? How do I know if a prediction is good or not?
Thank you !