Stochastic Volatility model

In [1]:
import numpy as np
import pymc3 as pm
from pymc3.distributions.timeseries import GaussianRandomWalk

from scipy import optimize

%pylab inline
Populating the interactive namespace from numpy and matplotlib

Asset prices have time-varying volatility (variance of day over day returns). In some periods, returns are highly variable, while in others very stable. Stochastic volatility models model this with a latent volatility variable, modeled as a stochastic process. The following model is similar to the one described in the No-U-Turn Sampler paper, Hoffman (2011) p21.

\[\sigma \sim Exponential(50)\]
\[\nu \sim Exponential(.1)\]
\[s_i \sim Normal(s_{i-1}, \sigma^{-2})\]
\[log(\frac{y_i}{y_{i-1}}) \sim t(\nu, 0, exp(-2 s_i))\]

Here, \(y\) is the daily return series and \(s\) is the latent log volatility process.

Build Model

First we load some daily returns of the S&P 500.

In [2]:
n = 400
returns = np.genfromtxt(pm.get_data("SP500.csv"))[-n:]
array([-0.00637 , -0.004045, -0.02547 ,  0.005102, -0.047733])
In [3]:
[<matplotlib.lines.Line2D at 0x11a62f198>]

Specifying the model in pymc3 mirrors its statistical specification.

In [4]:
model = pm.Model()
with model:
    sigma = pm.Exponential('sigma', 1./.02, testval=.1)

    nu = pm.Exponential('nu', 1./10)
    s = GaussianRandomWalk('s', sigma**-2, shape=n)

    r = pm.StudentT('r', nu, lam=pm.math.exp(-2*s), observed=returns)

Fit Model

For this model, the full maximum a posteriori (MAP) point is degenerate and has infinite density. To get good convergence with NUTS we use ADVI (autodiff variational inference) for initialization.

In [8]:
with model:
    trace = pm.sample(1000, tune=1000)
Auto-assigning NUTS sampler...
Initializing NUTS using ADVI...
Average Loss = -1,130.4:  34%|███▍      | 67873/200000 [00:15<00:31, 4184.66it/s]
Convergence archived at 68100
Interrupted at 68,100 [34%]: Average Loss = 505.57
100%|██████████| 2000/2000 [01:44<00:00, 20.78it/s]
In [9]:
pm.traceplot(trace, model.vars[:-1]);
In [10]:
plot(trace[s].T, 'b', alpha=.03);
ylabel('log volatility')
<matplotlib.text.Text at 0x13368e0b8>

Looking at the returns over time and overlaying the estimated standard deviation we can see how the model tracks the volatility over time.

In [11]:
plot(np.exp(trace[s].T), 'r', alpha=.03);
sd = np.exp(trace[s].T)
ylabel('absolute returns')
<matplotlib.text.Text at 0x13d0a7208>