smoking: Feel free to check out the final notebook.
Lesson
You can get a substantially different conclusion to the same data if part of it is not taken into account. This is one of the things that makes working with data super hard to get right. Our saving grace here was critical thinking (always double-check).
There are many aspects that we might also want to check;
Do people stop smoking after a while, what is this effect?
Does the effect of smoking depend on gender?
Can we distinguish between heavy smokers and light ones?
We should be very careful. We don't want to be the one who is lying with statistics.