I have no idea about how to tackle a problem with 2 quantitative variable X and Y. How can i model $Y=f(X)$ ? What is $f$ if i just have a look of the data ? how can i even guess if such an $f$ exist ?
I feel like i have no key to determine a relationship and the kind of relationship between two variable... (and i don't find anything in Hasti Tibshirani 's book)
For example i try to manage a complete data project. One question asked is to "determine the dependance between two variable: "age" and "note" (grade)".
I have 2 quantitative variable so:
I first plot them (the colours distinguish the class variable: 0 and 1 but is not important here!) We can see that there isn't any linear correlation on the graph. Moreover i can't see any other kind of relationship (like log aor squared root, polynomial,...)
I didn't know what to do to "evaluate" the "dependance" between my two variable so i did a linear model and without any surprises the coefficient $R^2$ is really small wich indicate a really poor linear relationship.
I don't know what to do next ...
If you have any suggestions please share it.