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# Model Selection using lmfit and emcee

FIXME: this is a useful example; however, it doesn’t run correctly anymore as the PTSampler was removed in emcee v3…

lmfit.emcee can be used to obtain the posterior probability distribution of parameters, given a set of experimental data. This notebook shows how it can be used for Bayesian model selection.

    import matplotlib.pyplot as plt
    import numpy as np

    import lmfit

Define a Gaussian lineshape and generate some data:

    def gauss(x, a_max, loc, sd):
        return a_max * np.exp(-((x - loc) / sd)**2)

    x = np.linspace(3, 7, 250)
    np.random.seed(0)
    y = 4 + 10 * x + gauss(x, 200, 5, 0.5) + gauss(x, 60, 5.8, 0.2)
    dy = np.sqrt(y)
    y += dy * np.random.randn(y.size)

Plot the data:

    plt.errorbar(x, y)

Define the normalised residual for the data:

    def residual(p, just_generative=False):
        v = p.valuesdict()
        generative = v['a'] + v['b'] * x
        M = 0
        while f'a_max{M}' in v:
            generative += gauss(x, v[f'a_max{M}'], v[f'loc{M}'], v[f'sd{M}'])
            M += 1

        if just_generative:
            return generative
        return (generative - y) / dy

Create a Parameter set for the initial guesses:

    def initial_peak_params(M):
        p = lmfit.Parameters()

        # a and b give a linear background
        a = np.mean(y)
        b = 1

        # a_max, loc and sd are the amplitude, location and SD of each Gaussian
        # component
        a_max = np.max(y)
        loc = np.mean(x)
        sd = (np.max(x) - np.min(x)) * 0.5

        p.add_many(('a', a, True, 0, 10), ('b', b, True, 1, 15))

        for i in range(M):
            p.add_many((f'a_max{i}', 0.5 * a_max, True, 10, a_max),
                       (f'loc{i}', loc, True, np.min(x), np.max(x)),
                       (f'sd{i}', sd, True, 0.1, np.max(x) - np.min(x)))
        return p

Solving with minimize gives the Maximum Likelihood solution.

    p1 = initial_peak_params(1)
    mi1 = lmfit.minimize(residual, p1, method='differential_evolution')

    lmfit.printfuncs.report_fit(mi1.params, min_correl=0.5)

From inspection of the data above we can tell that there is going to be more than 1 Gaussian component, but how many are there? A Bayesian approach can be used for this model selection problem. We can do this with lmfit.emcee, which uses the emcee package to do a Markov Chain Monte Carlo sampling of the posterior probability distribution. lmfit.emcee requires a function that returns the log-posterior probability. The log-posterior probability is a sum of the log-prior probability and log-likelihood functions.

The log-prior probability encodes information about what you already believe about the system. lmfit.emcee assumes that this log-prior probability is zero if all the parameters are within their bounds and -np.inf if any of the parameters are outside their bounds. As such it’s a uniform prior.

The log-likelihood function is given below. To use non-uniform priors then should include these terms in lnprob. This is the log-likelihood probability for the sampling.

    def lnprob(p):
        resid = residual(p, just_generative=True)
        return -0.5 * np.sum(((resid - y) / dy)**2 + np.log(2 * np.pi * dy**2))

To start with we have to create the minimizers and burn them in. We create 4 different minimizers representing 0, 1, 2 or 3 Gaussian contributions. To do the model selection we have to integrate the over the log-posterior distribution to see which has the higher probability. This is done using the thermodynamic_integration_log_evidence method of the sampler attribute contained in the lmfit.Minimizer object.

    # Work out the log-evidence for different numbers of peaks:
    total_steps = 310
    burn = 300
    thin = 10
    ntemps = 15
    workers = 1  # the multiprocessing does not work with sphinx-gallery
    log_evidence = []
    res = []

    # set up the Minimizers
    for i in range(4):
        p0 = initial_peak_params(i)
        # you can't use lnprob as a userfcn with minimize because it needs to be
        # maximised
        mini = lmfit.Minimizer(residual, p0)
        out = mini.minimize(method='differential_evolution')
        res.append(out)

    mini = []
    # burn in the samplers
    for i in range(4):
        # do the sampling
        mini.append(lmfit.Minimizer(lnprob, res[i].params))
        out = mini[i].emcee(steps=total_steps, ntemps=ntemps, workers=workers,
                            reuse_sampler=False, float_behavior='posterior',
                            progress=False)
        # get the evidence
        print(i, total_steps, mini[i].sampler.thermodynamic_integration_log_evidence())
        log_evidence.append(mini[i].sampler.thermodynamic_integration_log_evidence()[0])

Once we’ve burned in the samplers we have to do a collection run. We thin out the MCMC chain to reduce autocorrelation between successive samples.

    for j in range(6):
        total_steps += 100
        for i in range(4):
            # do the sampling
            res = mini[i].emcee(burn=burn, steps=100, thin=thin, ntemps=ntemps,
                                workers=workers, reuse_sampler=True, progress=False)
            # get the evidence
            print(i, total_steps, mini[i].sampler.thermodynamic_integration_log_evidence())
            log_evidence.append(mini[i].sampler.thermodynamic_integration_log_evidence()[0])

    plt.plot(log_evidence[-4:])
    plt.ylabel('Log-evidence')
    plt.xlabel('number of peaks')

The Bayes factor is related to the exponential of the difference between the log-evidence values. Thus, 0 peaks is not very likely compared to 1 peak. But 1 peak is not as good as 2 peaks. 3 peaks is not that much better than 2 peaks.

    r01 = np.exp(log_evidence[-4] - log_evidence[-3])
    r12 = np.exp(log_evidence[-3] - log_evidence[-2])
    r23 = np.exp(log_evidence[-2] - log_evidence[-1])

    print(r01, r12, r23)

These numbers tell us that zero peaks is 0 times as likely as one peak. Two peaks is 7e49 times more likely than one peak. Three peaks is 1.1 times more likely than two peaks. With this data one would say that two peaks is sufficient. Caution has to be taken with these values. The log-priors for this sampling are uniform but improper, i.e. they are not normalised properly. Internally the lnprior probability is calculated as 0 if all parameters are within their bounds and -np.inf if any parameter is outside the bounds. The lnprob function defined above is the log-likelihood alone. Remember, that the log-posterior probability is equal to the sum of the log-prior and log-likelihood probabilities. Extra terms can be added to the lnprob function to calculate the normalised log-probability. These terms would look something like:

\[\log (\prod_i \frac{1}{max_i - min_i})\]

where \(max_i\) and \(min_i\) are the upper and lower bounds for the parameter, and the prior is a uniform distribution. Other types of prior are possible. For example, you might expect the prior to be Gaussian.

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