I have a large data frame, df, containing 4 columns:
id period ret_1m mkt_ret_1m
131146 CAN00WG0 199609 -0.1538 0.047104
133530 CAN00WG0 199610 -0.0455 -0.014143
135913 CAN00WG0 199611 0.0000 0.040926
138334 CAN00WG0 199612 0.2952 0.008723
140794 CAN00WG0 199701 -0.0257 0.039916
143274 CAN00WG0 199702 -0.0038 -0.025442
145754 CAN00WG0 199703 -0.2992 -0.049279
148246 CAN00WG0 199704 -0.0919 -0.005948
150774 CAN00WG0 199705 0.0595 0.122322
153318 CAN00WG0 199706 -0.0337 0.045765
id period ret_1m mkt_ret_1m
160980 CAN00WH0 199709 0.0757 0.079293
163569 CAN00WH0 199710 -0.0741 -0.044000
166159 CAN00WH0 199711 0.1000 -0.014644
168782 CAN00WH0 199712 -0.0909 -0.007072
171399 CAN00WH0 199801 -0.0100 0.001381
174022 CAN00WH0 199802 0.1919 0.081924
176637 CAN00WH0 199803 0.0085 0.050415
179255 CAN00WH0 199804 -0.0168 0.018393
181880 CAN00WH0 199805 0.0427 -0.051279
184516 CAN00WH0 199806 -0.0656 -0.011516
id period ret_1m mkt_ret_1m
143275 CAN00WO0 199702 -0.1176 -0.025442
145755 CAN00WO0 199703 -0.0074 -0.049279
148247 CAN00WO0 199704 -0.0075 -0.005948
150775 CAN00WO0 199705 0.0451 0.122322
etc.
I am attempting to calculate a common financial measure, known as beta, using a function, that takes two of the columns, ret_1m, the monthly stock_return, and ret_1m_mkt, the market 1 month return for the same period (period_id). I want to apply a function (calc_beta) to calculate the 12-month result of this function on a 12 month rolling basis.
To do this, I am creating a groupby object:
grp = df.groupby('id')
What I would like to do is use something like:
period = 12
for stock, sub_df in grp:
arg = sub_df[['ret_1m', 'mkt_ret_1m']]
beta = pd.rolling_apply(arg, period, calc_beta, min_periods = period)
Now, here is the first problem. According to the documentation, pd.rolling_apply arg can be either a series or a data frame. However, it appears that the data frame I supply is converted into a numpy array that can only contain one column of data, rather than the two I have tried to supply. So my code below for calc_beta will not work, because I need to pass both the stock and market returns:
def calc_beta(np_array)
s = np_array[:,0] # stock returns are column zero from numpy array
m = np_array[:,1] # market returns are column one from numpy array
covariance = np.cov(s,m) # Calculate covariance between stock and market
beta = covariance[0,1]/covariance[1,1]
return beta
So my questions are as follows, I think it makes sense to list them in this way:
(i) How can I pass a data frame/multiple series/numpy array with more than one column to calc_beta using rolling_apply?
(ii) How can I return more than one value (e.g. the beta) from the calc_beta function?
(iii) Having calculated rolling quantities, how can I recombined with the original dataframe df so that I have the rolling quantities corresponding to the correct date in the period column?
(iv) Is there a better (vectorized) way of achieving this? I have seen some similar questions using e.g. df.apply(pd.rolling_apply,period,??) but I did not understand how these worked.
I gather that rolling_apply previously was unable to handle data frames, but the documentations suggests that it is now able to do so. My pandas.version is 0.16.1.
Thanks for any help! I have lost 1.5 days trying to figure this out and am totally stumped.
Ultimately, what I want is something like this:
id period ret_1m mkt_ret_1m beta other_quantities
131146 CAN00WG0 199609 -0.1538 0.047104 0.521 xxx
133530 CAN00WG0 199610 -0.0455 -0.014143 0.627 xxxx
135913 CAN00WG0 199611 0.0000 0.040926 0.341 xxx
138334 CAN00WG0 199612 0.2952 0.008723 0.567 xx
140794 CAN00WG0 199701 -0.0257 0.039916 0.4612 xxx
143274 CAN00WG0 199702 -0.0038 -0.025442 0.215 xxx
145754 CAN00WG0 199703 -0.2992 -0.049279 0.4678 xxx
148246 CAN00WG0 199704 -0.0919 -0.005948 -0.4225 xxx
150774 CAN00WG0 199705 0.0595 0.122322 0.780 xxx
153318 CAN00WG0 199706 -0.0337 0.045765 0.623 xxx
id period ret_1m mkt_ret_1m beta other_quantities
160980 CAN00WH0 199709 0.0757 0.079293 -0.913 xx
163569 CAN00WH0 199710 -0.0741 -0.044000 0.894 xxx
166159 CAN00WH0 199711 0.1000 -0.014644 0.563 xxx
168782 CAN00WH0 199712 -0.0909 -0.007072 0.734 xxx
171399 CAN00WH0 199801 -0.0100 0.001381 0.894 xxxx
174022 CAN00WH0 199802 0.1919 0.081924 0.789 xx
176637 CAN00WH0 199803 0.0085 0.050415 0.1563 xxxx
179255 CAN00WH0 199804 -0.0168 0.018393 -0.64 xxxx
181880 CAN00WH0 199805 0.0427 -0.051279 -0.742 xxx
184516 CAN00WH0 199806 -0.0656 -0.011516 0.925 xxx
id period ret_1m mkt_ret_1m beta
143275 CAN00WO0 199702 -0.1176 -0.025442 -1.52 xx
145755 CAN00WO0 199703 -0.0074 -0.049279 -0.632 xxx
148247 CAN00WO0 199704 -0.0075 -0.005948 1.521 xx
150775 CAN00WO0 199705 0.0451 0.122322 0.0321 xxx
etc.
