Very often, we are interested in tallying the results from a table. That is, we are interested in counting how many instances of some kind of observation are within our table.
Pandas makes counting of values relatively easy.
An important step in tallying is to figure out the unique values within the collection of items that you are tallying. For example, in the data we have been looking at:
df['state'].unique()
Will give you an array containing the unique values of states.
df['state'].value_counts()
Will give you the number of individuals in each bin.
Note that this is also a Series object, so if you want to do further calculations with these
numbers you can do that too.
Cross-tabs are tables that display the contingent distributions of frequencies of certain combinations of variable values. For example, we can calculate the frequency of gender by state:
ctab_state_gender = df.groupby(["state", "gender"]).count()
The result of cross-tabulation is also a DataFrame, but it has a more complicated index than
the DataFrame objects we have seen so far.
This is called a MultiIndex:
ctab_state_gender
MultiIndex(levels=[['act', 'aw', 'nss', 'nsw', 'nt', 'nws', 'qdl', 'qld', 'qod', 'sa', 'sk', 'tas', 'vic', 'vid', 'vif', 'viy', 'vkc', 'wa', 'wz'], ['None', 'female', 'male']],
labels=[[0, 0, 0, 1, 2, 3, 3, 3, 4, 4, 5, 6, 7, 7, 7, 8, 9, 9, 9, 10, 11, 11, 11, 12, 12, 12, 13, 14, 15, 16, 17, 17, 17, 18], [0, 1, 2, 2, 2, 0, 1, 2, 1, 2, 2, 2, 0, 1, 2, 1, 0, 1, 2, 1, 0, 1, 2, 0, 1, 2, 1, 0, 2, 1, 0, 1, 2, 2]],
names=['state', 'gender'])
Here, the levels of the index are all the combinations between the first list and the second list within the levels
MultiIndex objects can be used in a similar way to they way that a regular Index is used.
For example, indexing using loc on the first level:
ctab_state_gender.loc["aw"]
will give you the table for that State (what would you do to get the table for “male” across states?)
You can also get a particular row in this table, by indexing with a sequence:
ctab_state_gender.loc["aw", "male"]
Create a cross-tab of tax brackets as a function of state.
How would you go about reporting a count of tax bracket, by state and gender?