1. Define a function that returns the percentage of schools with an undergraduate population between 1,000 and 3,000:

>>> def pct_between_1_3k(s):
        return s.between(1000, 3000).mean()

2. Calculate this percentage grouping by state and religious affiliation:

>>> college.groupby(['STABBR', 'RELAFFIL'])['UGDS'] 
           .agg(pct_between_1_3k).head(9)
STABBR  RELAFFIL
AK      0           0.142857
        1           0.000000
AL      0           0.236111
        1           0.333333
AR      0           0.279412
        1           0.111111
AS      0           1.000000
AZ      0           0.096774
        1           0.000000
Name: UGDS, dtype: float64

3. This function works fine but it doesn't give the user any flexibility to choose the lower and upper bound. Let's create a new function that allows the user to define these bounds:

>>> def pct_between(s, low, high):
        return s.between(low, high).mean()

4. Pass this new function to the agg method along with lower and upper bounds:

>>> college.groupby(['STABBR', 'RELAFFIL'])['UGDS'] 
           .agg(pct_between, 1000, 10000).head(9)
STABBR  RELAFFIL
AK      0           0.428571
        1           0.000000
AL      0           0.458333
        1           0.375000
AR      0           0.397059
        1           0.166667
AS      0           1.000000
AZ      0           0.233871
        1           0.111111
Name: UGDS, dtype: float64