Calculating weighted mean SAT scores per state with apply

The groupby object has four methods that accept a function (or functions) to perform a calculation on each group. These four methods are agg, filter, transform, and apply. Each of the first three of these methods has a very specific output that the function must return. agg must return a scalar value, filter must return a boolean, and transform must return a Series with the same length as the passed group. The apply method, however, may return a scalar value, a Series, or even a DataFrame of any shape, therefore making it very flexible. It is also called only once per group, which contrasts with transform and agg that get called once for each non-grouping column. The apply method's ability to return a single object when operating on multiple columns at the same time makes the calculation in this recipe possible.

Getting ready

In this recipe, we calculate the weighted average of both the math and verbal SAT scores per state from the...