```
In [ ]:
```%load_ext autoreload
%autoreload 2
from __future__ import print_function
import numpy as np
import matplotlib.pyplot as plt
%matplotlib inline
plt.rcParams['figure.figsize'] = (6.0, 6.0)
plt.rcParams['savefig.dpi'] = 100

```
In [ ]:
```from straightline_utils import *

```
In [ ]:
```(x,y,sigmay) = generate_data()
plot_yerr(x, y, sigmay)

```
In [ ]:
```def straight_line_log_likelihood(theta, x, y, sigmay):
'''
Returns the log-likelihood of drawing data values *y* at
known values *x* given Gaussian measurement noise with standard
deviation with known *sigmay*, where the "true" y values are
*y_t = m * x + b*
x: list of x coordinates
y: list of y coordinates
sigmay: list of y uncertainties
m: scalar slope
b: scalar line intercept
Returns: scalar log likelihood
'''
m,b = theta
return (np.sum(np.log(1./(np.sqrt(2.*np.pi) * sigmay))) +
np.sum(-0.5 * (y - (m*x + b))**2 / sigmay**2))

```
In [ ]:
```def straight_line_log_prior(theta, theta_limits):
m, b = theta
mlimits, blimits = theta_limits
# Uniform in m:
if (m < mlimits[0]) | (m > mlimits[1]):
log_m_prior = -np.inf
else:
log_m_prior = np.log(1.0/(mlimits[1] - mlimits[0]))
# Uniform in b:
if (b < blimits[0]) | (b > blimits[1]):
log_b_prior = -np.inf
else:
log_b_prior = np.log(1.0/(blimits[1] - blimits[0]))
return log_m_prior + log_b_prior
def straight_line_log_posterior(theta, x, y, sigmay, theta_limits):
return (straight_line_log_likelihood(theta, x, y, sigmay) +
straight_line_log_prior(theta, theta_limits))

```
In [ ]:
```# Evaluate log P(m,b | x,y,sigmay) on a grid.
# Define uniform prior limits, enforcing positivity in both parameters:
mlimits = [0.0, 2.0]
blimits = [0.0, 200.0]
theta_limits = (mlimits, blimits)
# Set up grid:
mgrid = np.linspace(mlimits[0], mlimits[1], 101)
bgrid = np.linspace(blimits[0], blimits[1], 101)
log_posterior = np.zeros((len(mgrid),len(bgrid)))
# Evaluate log posterior PDF:
for im,m in enumerate(mgrid):
for ib,b in enumerate(bgrid):
theta = (m,b)
log_posterior[im,ib] = straight_line_log_posterior(theta, x, y, sigmay, theta_limits)
# Convert to probability density and plot, taking care with very small values:
posterior = np.exp(log_posterior - log_posterior.max())
plt.imshow(posterior, extent=[blimits[0],blimits[1],mlimits[0],mlimits[1]],cmap='Blues',
interpolation='none', origin='lower', aspect=(blimits[1]-blimits[0])/(mlimits[1]-mlimits[0]),
vmin=0, vmax=1)
plt.contour(bgrid, mgrid, posterior, pdf_contour_levels(posterior), colors='k')
i = np.argmax(posterior)
i,j = np.unravel_index(i, posterior.shape)
print('Grid maximum posterior values (m,b) =', mgrid[i], bgrid[j])
plt.title('Straight line: posterior PDF for parameters');
plot_mb_setup(*theta_limits);

- And now to draw some samples:

```
In [ ]:
```def metropolis(log_posterior, theta, theta_limits, stepsize, nsteps=10000):
'''
log_posterior: function of theta
theta_limits: uniform prior ranges
stepsize: scalar or vector proposal distribution width
nsteps: desired number of samples
'''
log_prob = log_posterior(theta, x, y, sigmay, theta_limits)
# Store Markov chain as an array of samples:
chain = np.empty((nsteps, len(theta)))
log_probs = np.empty(nsteps)
# Count our accepted proposals:
naccept = 0
for i in range(nsteps):
theta_new = theta + stepsize * np.random.randn(len(theta))
log_prob_new = log_posterior(theta_new, x, y, sigmay, theta_limits)
if np.log(np.random.rand()) < (log_prob_new - log_prob):
# accept, and move to the proposed position:
theta = theta_new
log_prob = log_prob_new
naccept += 1
else:
# reject, and store the same sample as before:
pass
chain[i] = theta
log_probs[i] = log_prob
acceptance_rate = naccept/float(nsteps)
return chain,log_probs,acceptance_rate

```
In [ ]:
```# Initialize m, b at the center of prior:
m = 0.5*(mlimits[0]+mlimits[1])
b = 0.5*(blimits[0]+blimits[1])
theta = np.array([m,b])
# Step sizes, 2% or 5% of the prior
mstep = 0.02*(mlimits[1]-mlimits[0])
bstep = 0.05*(blimits[1]-blimits[0])
stepsize = np.array([mstep,bstep])
# How many steps?
nsteps = 10000
print('Running Metropolis Sampler for', nsteps, 'steps...')
chain, log_probs, acceptance_rate = metropolis(
straight_line_log_posterior, theta, theta_limits, stepsize, nsteps=nsteps
)
print('Acceptance fraction:', acceptance_rate)

```
In [ ]:
```# Pull m and b arrays out of the Markov chain and plot them:
mm = [m for m,b in chain]
bb = [b for m,b in chain]
# Traces, for convergence inspection:
plt.figure(figsize=(8,5))
plt.subplot(2,1,1)
plt.plot(mm, 'k-')
plt.ylim(mlimits)
plt.ylabel('m')
plt.subplot(2,1,2)
plt.plot(bb, 'k-')
plt.ylabel('Intercept b')
plt.ylim(blimits)

- Looks reasonable: a short burn-in period, followed by reasonably well-mixed samples.

- Now let's look at the samples in parameter space, overlaid on our gridded posterior.

```
In [ ]:
```!pip install --upgrade --no-deps corner

```
In [ ]:
```import corner
corner.corner(chain[5000:], labels=['m','b'], range=[mlimits,blimits],quantiles=[0.16,0.5,0.84],
show_titles=True, title_args={"fontsize": 12},
plot_datapoints=True, fill_contours=True, levels=[0.68, 0.95],
color='b', bins=80, smooth=1.0);

- It looks like we made a nice, precise measurement!

- We made a lot of assumptions though - so we now need to test them. Our next step is to check the model for accuracy.

- How do we know if our model is any good? One property that "good" models have is
*accuracy.*

- Accurate models generate data that is
*like*the observed data. What does this mean? First we have to define what similarity is, in this context.

*Visual impression*is one very important way, that's usually best done first.

*Test statistics*that capture relevant features of the data are another. It's good to explore the posterior predictive distribution for these statistics, as a way of propagating our uncertainty in the model parameters into our predictions.

```
In [ ]:
```# Generate a straight line model for each parameter combination, and plot:
X = np.linspace(xlimits[0],xlimits[1],50)
for i in (np.random.rand(100)*len(chain)).astype(int):
m,b = chain[i]
plt.plot(X, b+X*m, 'b-', alpha=0.1)
# Overlay the data, for comparison:
plot_yerr(x, y, sigmay);

- If our model is the true one, then
*replica*data generated by it should look like the one dataset we have. This means that*summaries*of both the real dataset, $T(d)$, and the replica datasets, $T(d^{\rm rep})$, should be drawn from the same distribution.

- If the real dataset was not generated with our model, then its summary may be an outlier from the distribution of summaries of replica datasets.

- We can account for our uncertainty in the parameters $\theta$ by marginalizing them out, which can be easily done by just making the histogram of $T(d^{\rm rep}(\theta))$ from our posterior samples, after drawing one replica dataset $d^{\rm rep}$ from the model sampling distribution ${\rm Pr}(d^{\rm rep}\,|\,\theta)$ for each one.

- Then, we can ask: what is the posterior probability for the summary $T$ to be greater than the observed summary $T(d)$? If this is very small or very large, we should be suspicious of our model - because it is not predicting the data very accurately.

${\rm Pr}(T(d^{\rm rep})>T(d)\,|\,d) = \int I(T(d^{\rm rep})>T(d))\,{\rm Pr}(d^{\rm rep}\,|\,\theta)\,{\rm Pr}(\theta\,|\,d)\;d\theta\,dd^{\rm rep}$

- Here $I$ is the "indicator function" - 1 or 0 according to the condition.

```
In [ ]:
```# Test statistics: functions of the data, not the parameters.
# 1) Weighted mean y:
# def test_statistic(x,y,sigmay):
# return np.sum(y/sigmay**2.0)/np.sum(1.0/sigmay**2.0)
# 2) Variance of y:
# def test_statistic(x,y,sigmay):
# return np.var(y)
# 3) Pearson r correlation coefficient:
import scipy.stats
def test_statistic(x,y,dummy):
'''
Pearson r correlation coefficient:
r12 = \sum [(xi - xbar)*(yi - ybar)] / [\sum (xi - xbar)^2 * \sum (yi - ybar)^2]^1/2
'''
r12 = np.sum((x - np.mean(x))*(y - np.mean(y))) / \
np.sqrt(np.sum((x - np.mean(x))**2) * np.sum((y - np.mean(y))**2))
return r12
# Approximate the posterior predictive distribution for T,
# by drawing a replica dataset for each sample (m,b) and computing its T:
T = np.zeros(len(chain))
for k,(m,b) in enumerate(chain):
yrep = b + m*x + np.random.randn(len(x)) * sigmay
T[k] = test_statistic(x,yrep,sigmay)
# Compare with the test statistic of the data, on a plot:
Td = test_statistic(x, y, sigmay)
plt.hist(T, 100, histtype='step', color='blue', lw=2, range=(0.0,np.percentile(T,99.0)))
plt.axvline(Td, color='black', linestyle='--', lw=2)
plt.xlabel('Test statistic')
plt.ylabel('Posterior predictive distribution')
# What is Pr(T>T(d)|d)?
greater = (T > Td)
P = 100*len(T[greater])/(1.0*len(T))
print("Pr(T>T(d)|d) = ",P,"%")

- If our model is true (and we're just uncertain about its parameters, given the data), we can compute the probability of getting a $T$ less than that observed.

- Note that we did not have to look up any particular standard distribution - we can simply compute the posterior predictive distribution given our generative model.

- This test statistic lacks power: better choices might put more acute stress on the model to perform, by focusing on the places where the model predictions are suspect.

- Test statistics $T(d,\theta)$ that are functions of both the data and the parameters are known as
*discrepancy measures.*

- Similar in spirit to the above, we can compute the posterior probability of getting $T(d^{\rm rep},\theta) > T(d,\theta)$:

${\rm Pr}(T(d^{\rm rep},\theta)>T(d,\theta)\,|\,d) = \int I(T(d^{\rm rep},\theta)>T(d,\theta))\,{\rm Pr}(d^{\rm rep}\,|\,\theta)\,{\rm Pr}(\theta\,|\,d)\;d\theta\,dd^{\rm rep}$

```
In [ ]:
```# Discrepancy: functions of the data AND parameters.
# 1) Reduced chisq for the model:
def discrepancy(x,y,sigmay,b,m):
return np.sum((y - m*x - b)**2.0/sigmay**2.0)/(len(y)-2)
# Approximate the posterior predictive distribution for T,
# by drawing a replica dataset for each sample (m,b) and computing its T,
# AND ALSO its Td (which now depends on the parameters, too):
T = np.zeros(len(chain))
Td = np.zeros(len(chain))
for k,(m,b) in enumerate(chain):
yrep = b + m*x + np.random.randn(len(x)) * sigmay
T[k] = discrepancy(x,yrep,sigmay,b,m)
Td[k] = discrepancy(x,y,sigmay,b,m)
# Compare T with Td, on a scatter plot - how often is T>Td?
plt.scatter(Td, T, color='blue',alpha=0.1)
plt.plot([0.0, 100.0], [0.0, 100.], color='k', linestyle='--', linewidth=2)
plt.xlabel('Observed discrepancy $T(d,\\theta)$')
plt.ylabel('Replicated discrepancy $T(d^{\\rm rep},\\theta)$')
plt.ylim([0.0,np.percentile(Td,99.0)])
plt.xlim([0.0,np.percentile(Td,99.0)])

```
In [ ]:
```# Histogram of differences:
diff = T-Td
plt.hist(diff, 100, histtype='step', color='blue', lw=2, range=(np.percentile(diff,1.0),np.percentile(diff,99.0)))
plt.axvline(0.0, color='black', linestyle='--', lw=2)
plt.xlabel('Difference $T(d^{\\rm rep},\\theta) - T(d,\\theta)$')
plt.ylabel('Posterior predictive distribution')
# What is Pr(T>T(d)|d)?
greater = (T > Td)
Pline = 100*len(T[greater])/(1.0*len(T))
print("Pr(T>T(d)|d) = ",Pline,"%")

- The conclusion drawn from the discrepancy is more interesting, in this case. All our $\theta = (m,b)$ samples are plausible, so replica datasets generated by them should also be plausible. The straight line defined by each $(m,b)$ should go through the real data points as readily (
*on average*) as it does its replica dataset.

- Do our posterior predictive $p-$values suggest we need to improve our model? What about the visual check?

In some sense, the reduced chi-squared is actually not such an interesting test statistic, because it's very similar to the log likelihood!

Still more powerful discrepancy measures might stress-test

*different*aspects of the model. Talk to your neighbor about where and how this model might be failing, and see if you can design a better discrepancy measure than reduced chi-squared.

- Maybe I see some curvature in the data - or maybe I have a new astrophysical idea for how this data was generated. Let's try adding an extra parameter to the model, to make a quadratic function:

$y = m x + b + q x^2$

- The coefficient $q$ is probably pretty small (we were originally expecting to only have to use a straight line model for these data!), so I guess we can assign a fairly narrow prior, centered on zero.

```
In [ ]:
```def quadratic_log_likelihood(theta, x, y, sigmay):
'''
Returns the log-likelihood of drawing data values y at
known values x given Gaussian measurement noise with standard
deviation with known sigmay, where the "true" y values are
y_t = m*x + b + q**2
x: list of x coordinates
y: list of y coordinates
sigmay: list of y uncertainties
m: scalar slope
b: scalar line intercept
q: quadratic term coefficient
where theta = (m, b, q)
Returns: scalar log likelihood
'''
m, b, q = theta
return (np.sum(np.log(1./(np.sqrt(2.*np.pi) * sigmay))) +
np.sum(-0.5 * (y - (m*x + b + q*x**2))**2 / sigmay**2))
def quadratic_log_prior(theta, theta_limits):
m, b, q = theta
mlimits, blimits, qpars = theta_limits
# m and b:
log_mb_prior = straight_line_log_prior(np.array([m,b]), np.array([mlimits, blimits]))
# q:
log_q_prior = np.log(1./(np.sqrt(2.*np.pi) * qpars[1])) - \
0.5 * (q - qpars[0])**2 / qpars[1]**2
return log_mb_prior + log_q_prior
def quadratic_log_posterior(theta, x, y, sigmay, theta_limits):
return (quadratic_log_likelihood(theta, x, y, sigmay) +
quadratic_log_prior(theta, theta_limits))

```
In [ ]:
```# Define uniform prior limits, enforcing positivity in m and b:
mlimits = [0.0, 2.0]
blimits = [0.0, 200.0]
# Define Gaussian prior centered on zero for q:
qpars = [0.0,0.003]
# Initialize m, b at the center of prior:
m = 0.5*(mlimits[0]+mlimits[1])
b = 0.5*(blimits[0]+blimits[1])
q = qpars[0]
# Arrays to pass to the sampler:
qtheta = np.array([m,b,q])
qtheta_limits = (mlimits, blimits, qpars)
# Step sizes, small fractions of the prior width:
mstep = 0.01*(mlimits[1]-mlimits[0])
bstep = 0.04*(blimits[1]-blimits[0])
qstep = 0.02*qpars[1]
stepsize = np.array([mstep,bstep,qstep])
# How many steps?
nsteps = 10000
print('Running Metropolis Sampler for', nsteps, 'steps...')
qchain, log_probs, acceptance_rate = metropolis(
quadratic_log_posterior, qtheta, qtheta_limits, stepsize, nsteps=nsteps
)
print('Acceptance fraction:', acceptance_rate)

```
In [ ]:
```# Pull m, b and q arrays out of the Markov chain and plot them:
mm = [m for m,b,q in qchain]
bb = [b for m,b,q in qchain]
qq = [q for m,b,q in qchain]
# Traces, for convergence inspection:
plt.figure(figsize=(8,5))
plt.subplot(3,1,1)
plt.plot(mm, 'k-')
plt.ylim(mlimits)
plt.ylabel('Slope m')
plt.subplot(3,1,2)
plt.plot(bb, 'k-')
plt.ylim(blimits)
plt.ylabel('Intercept b')
plt.subplot(3,1,3)
plt.plot(qq, 'k-')
plt.ylim([qpars[0]-3*qpars[1],qpars[0]+3*qpars[1]])
plt.ylabel('Quadratic coefficient q')

```
In [ ]:
```corner.corner(qchain, labels=['m','b','q'], range=[mlimits,blimits,(qpars[0]-3*qpars[1],qpars[0]+3*qpars[1])],quantiles=[0.16,0.5,0.84],
show_titles=True, title_args={"fontsize": 12},
plot_datapoints=True, fill_contours=True, levels=[0.68, 0.95],
color='green', bins=80, smooth=1.0);

- All parameters are again precisely constrained.

- The gradient and intercept $m$ and $b$ are significantly different from before, though...

```
In [ ]:
```# Posterior visual check, in data space:
X = np.linspace(xlimits[0],xlimits[1],100)
for i in (np.random.rand(100)*len(chain)).astype(int):
m,b,q = qchain[i]
plt.plot(X, b + X*m + q*X**2, 'g-', alpha=0.1)
plot_yerr(x, y, sigmay)

```
In [ ]:
```# Discrepancy: functions of the data AND parameters.
# 1) Reduced chisq for the model:
def discrepancy(x,y,sigmay,m,b,q):
return np.sum((y - m*x - b - q*x**2)**2.0/sigmay**2.0)/(len(y)-3)
# Approximate the posterior predictive distribution for T,
# by drawing a replica dataset for each sample (m,b) and computing its T,
# AND ALSO its Td:
T = np.zeros(len(qchain))
Td = np.zeros(len(qchain))
for k,(m,b,q) in enumerate(qchain):
yp = b + m*x + q*x**2 + sigmay*np.random.randn(len(x))
T[k] = discrepancy(x,yp,sigmay,m,b,q)
Td[k] = discrepancy(x,y,sigmay,m,b,q)
# Histogram of differences:
diff = T - Td
plt.hist(diff, 100, histtype='step', color='green', lw=2, range=(np.percentile(diff,1.0),np.percentile(diff,99.0)))
plt.axvline(0.0, color='black', linestyle='--', lw=2)
plt.xlabel('Difference $T(d^{\\rm rep},\\theta) - T(d,\\theta)$')
plt.ylabel('Posterior predictive distribution')
# What is Pr(T>T(d)|d)?
greater = (T > Td)
Pquad = 100*len(T[greater])/(1.0*len(T))
print("Pr(T>T(d)|d,quadratic) = ",Pquad,"%, cf. Pr(T>T(d)|d,straightline) = ",Pline,"%")

- How do the two models compare? Which one matches the data better?

- In the posterior predictive check above we found it useful to consider the probability distribution of hypothetical replica datasets.

- As well as this, we also have access to the quantity ${\rm Pr}(d\,|\,H)$, the evidence for the model $H$. Let's take a look at its properties.

- The FML is in general quite difficult to calculate, since it involves averaging the likelihood over the prior. MCMC gives us samples from the posterior - and these cannot, it turns out, be reprocessed so as to estimate the evidence stably.

A number of sampling algorithms have been developed that

*do*calculate the evidence, during the process of sampling. These include:- Nested Sampling (including MultiNest and DNest)
- Parallel Tempering, Thermodynamic Integration
- ...

- If we draw samples from the
*prior*, we can then estimate the evidence via the usual sum over samples,

${\rm Pr}(d\,|\,H) \approx \frac{1}{N_s} \sum_k\;{\rm Pr}(d\,|\,\theta_s,H)$

## Simple Monte Carlo:

Consider the simplest possbile posterior inference, the mean of parameter $\theta$: $\bar{\theta} = \int\,\theta\,{\rm Pr}(\theta\,|\,d,H)\,d\theta$

This is approximated by an average over posterior samples:

$\bar{\theta} \approx \frac{1}{N_s}\,\sum_s\,\theta_s$

Now replace the posterior PDF with Bayes' Theorem:

$\bar{\theta} = \frac{1}{Z} \int\,\theta\,{\rm Pr}(d\,|\,\theta,H)\,{\rm Pr}(\theta\,|\,H)\,d\theta$

and the posterior samples with prior samples:

$\bar{\theta} \approx \frac{1}{(Z N_s)}\,\sum_s\,\theta_s\,{\rm Pr}(d\,|\,\theta_s,H)$

This is a

weighted mean- if we don't want $Z$, we just define weights $w_s = {\rm Pr}(d\,|\,\theta_s,H)$ compute the approximate posterior mean as the likelihood-weighted prior mean:$\bar{\theta} \approx \frac{\sum_s\,w_s\,\theta_s}{\sum_s\,w_s}$

To compute Z, we multiply both sides by $Z$ and replace $\theta$ with 1:

$Z \approx \frac{1}{N_s} \sum_s\,\,w_s$

- Simple Monte Carlo works well in certain low-dimensional situations, but in general it is very inefficient (at best).

- Still, let's give it a try on our two models, and attempt to compute the evidence ratio

$R = \frac{{\rm Pr}(d\,|\,{\rm quadratic})}{{\rm Pr}(d\,|\,{\rm straight line})}$

```
In [ ]:
```# Draw a large number of prior samples and calculate the log likelihood for each one:
N = 50000
# Set the priors:
mlimits = [0.0, 2.0]
blimits = [0.0, 200.0]
qpars = [0.0,0.003]
# Sample from the prior:
mm = np.random.uniform(mlimits[0],mlimits[1], size=N)
bb = np.random.uniform(blimits[0],blimits[1], size=N)
qq = qpars[0] + qpars[1]*np.random.randn(N)
# We'll store the posterior samples as a "chain" again
schain = []
log_likelihood_straight_line = np.zeros(N)
log_likelihood_quadratic = np.zeros(N)
for i in range(N):
theta = np.array([mm[i], bb[i]])
log_likelihood_straight_line[i] = straight_line_log_likelihood(theta, x, y, sigmay)
qtheta = np.array([mm[i], bb[i], qq[i]])
log_likelihood_quadratic[i] = quadratic_log_likelihood(qtheta, x, y, sigmay)
schain.append((mm[i],bb[i],qq[i]))

*not* want to plot the points as well as the contours, since the points are prior samples not posterior ones!):

```
In [ ]:
```# Unnormalized likelihoods for plotting:
unnormalized_likelihood_straight_line = np.exp(log_likelihood_straight_line - log_likelihood_straight_line.max())
unnormalized_likelihood_quadratic = np.exp(log_likelihood_quadratic - log_likelihood_quadratic.max())

```
In [ ]:
```corner.corner(schain, labels=['m','b','q'], range=[mlimits,blimits,(qpars[0]-3*qpars[1],qpars[0]+3*qpars[1])],quantiles=[0.16,0.5,0.84],
weights=unnormalized_likelihood_straight_line,
show_titles=True, title_args={"fontsize": 12},
plot_datapoints=False, fill_contours=True, levels=[0.68, 0.95],
color='blue', bins=80, smooth=1.0);

```
In [ ]:
```corner.corner(schain, labels=['m','b','q'], range=[mlimits,blimits,(qpars[0]-3*qpars[1],qpars[0]+3*qpars[1])],quantiles=[0.16,0.5,0.84],
weights=unnormalized_likelihood_quadratic,
show_titles=True, title_args={"fontsize": 12},
plot_datapoints=False, fill_contours=True, levels=[0.68, 0.95],
color='green', bins=80, smooth=1.0);

```
In [ ]:
```def logaverage(x):
mx = x.max()
return np.log(np.sum(np.exp(x - mx))) + mx - np.log(len(x))
log_evidence_straight_line = logaverage(log_likelihood_straight_line)
log_evidence_quadratic = logaverage(log_likelihood_quadratic)
print('log Evidence for Straight Line Model:', log_evidence_straight_line)
print('log Evidence for Quadratic Model:', log_evidence_quadratic)
print('Evidence ratio in favour of the Quadratic Model:', np.int(np.exp(log_evidence_quadratic - log_evidence_straight_line)),"to 1")

Play around with the Simple Monte Carlo evidence estimate, and see if you can break it.

Try running it a few times, with the same

`N`

: how precise are these evidence estimates from 50000 samples?We said we believed

*a priori*that $q$ should be small, and set the prior width accordingly. What would happen if we started to doubt ourselves, and re-assigned a broader prior? Try running with some larger values for the Gaussian prior width $q[1]$, and note what happens to the Bayes Factor.

- The Bayesian evidence is
*qualitatively different*from other model assessments. While they focus primarily on*prediction accuracy,*the evidence is the way in which information from the prior PDF propagates through into our posterior beliefs about the model as a whole.

- There are no mathematical limitations to its use[
*citation needed*], in contrast to various other hypothesis tests that are only valid under certain assumptions (such as the models being nested). Any two models can be compared and the odds ratio computed.

Good things to keep in mind when using, or reading about, the evidence for a model:

Garbage in, garbage out: if you don't believe your prior, why should you or anyone else believe your evidence?

The evidence is only linearly sensitive to prior volume, but exponentially sensitive to goodness of fit. If you want a clearer sense of which model makes the data more probable,

*get more data.*If you don't believe your priors, the evidence can be a distraction from things you care about more - like accuracy, or cost.

If you do believe your priors, the evidence is very useful: it appears in second-level inferences, when parts of your model that you previously considered to be constant now need to be varied and inferred. The FML is only one parameter liberation away from becoming a likelihood.

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