 # Intro Tutorial Linear Regression and Statistical Rethinking

Hi, I’m working my way through Statistical Rethinking with the adapted Pyro code. The code has been helpful so far, although the latest version of the textbook no longer matches up 1:1 with the adapted code, and some code is missing entirely.
Additionally, the function in the provided module rethinking.py that does quadratic approximation doesn’t work in my experience. Since I won’t be using quadratic approximation in practice, I figured I’d try to use SVI.
For the linear regression from Chapter 4 (model 4.3), I followed the Introduction to Pyro tutorial as closely as possible. However, the intercept parameter seems to be way off in the posterior check.

``````def model(weight=None, height=None):
a = pyro.sample("a", dist.Normal(160, 20))
b = pyro.sample("b", dist.Normal(0, 10))
sigma = pyro.sample("sigma", dist.Uniform(0, 50))

mu = a + b * weight

with pyro.plate('data'):
return pyro.sample("height", dist.Normal(mu, sigma), obs=height)

pyro.clear_param_store()

# These should be reset each training loop.
auto_guide = pyro.infer.autoguide.AutoNormal(model)
elbo = pyro.infer.Trace_ELBO()
svi = pyro.infer.SVI(model, auto_guide, adam, elbo)

losses = []
for step in range(1000):  # Consider running for more steps.
loss = svi.step(weight_c, height)
losses.append(loss)
if step % 100 == 0:
print("Elbo loss: {}".format(loss))

predictive = pyro.infer.Predictive(model, guide=auto_guide, num_samples=800)
svi_samples = predictive(weight_c, height=None)
svi_height = svi_samples["height"]

predictions = pd.DataFrame({
'weight': weight_c,
"h_mean": svi_height.mean(0).detach().cpu().numpy(),
"h_perc_5": svi_height.kthvalue(int(len(svi_height) * 0.05), dim=0).detach().cpu().numpy(),
"h_perc_95": svi_height.kthvalue(int(len(svi_height) * 0.95), dim=0).detach().cpu().numpy(),
'true_height': height,
}).sort_values(by='weight')

f, ax = plt.subplots(figsize=(12,8))

ax.plot(predictions['weight'], predictions['h_mean'])
ax.fill_between(predictions['weight'], predictions['h_perc_5'], predictions['h_perc_95'], alpha=0.5)

ax.plot(predictions['weight'], predictions['true_height'], "o")
ax.set(xlabel='weight (centered)', ylabel='height')
``````

where `weight_c` is the centered weight.

Slope (parameter `b`) seems correct, but the intercept (parameter `a`) seems way off (Normal(15,1) when it should be around Normal(160,20) as in my prior).

Additionally, it’s not clear to me how to get the variance-covariance matrix from this model as shown in the textbook.

Thoughts much appreciated.

hello. didn’t look at your code in detail but please see our tips and tricks. in particular take a look at sections 1 and 11. for example, you may want to reparameterize things to be order one, e.g.

``````a = 100.0 * pyro.sample("a_renormalized", dist.Normal(1.60, 0.20))
``````

Thanks. Changing the line as you suggested does seem to remedy the fitting issue (although it’s still a slight mystery to me why the misfit results the way it does).

Any thoughts re: getting the variance-covariance matrix as in the textbook?

i haven’t read and do not have time to read any textbook but if you formulate a specific question maybe someone can help

Question is how can I get the variance-covariance matrix from this model?

Question is how can I get the variance-covariance matrix from this model?

the covariance of what? the estimated posterior over the latent variables? something else?

Yes, the posterior over the latent variables.

`AutoNormal` uses a diagonal covariance matrix and so there are no covariances under this approximation. to compute variances you can use e.g. the quantiles method