The QLIM Procedure

Bayesian Analysis

To perform Bayesian analysis, you must specify a BAYES statement. Unless otherwise stated, all options in this section are options in the BAYES statement.

By default, PROC QLIM uses the random walk Metropolis algorithm to obtain posterior samples. For the implementation details of the Metropolis algorithm in PROC QLIM, such as the blocking of the parameters and tuning of the covariance matrices, see the sections Blocking of Parameters and Tuning the Proposal Distribution.

The Bayes theorem states that

\begin{equation*}  p(\theta | \mb {y})\propto \pi (\theta ) L(y|\theta ) \end{equation*}

where $\theta $ is a parameter or a vector of parameters and $\pi (\theta )$ is the product of the prior densities that are specified in the PRIOR statement. The term $L(y|\theta )$ is the likelihood associated with the MODEL statement.

Blocking of Parameters

In a multivariate parameter model, all the parameters are updated in one single block (by default or when you specify the SAMPLING=MULTIMETROPOLIS option). This could be inefficient, especially when parameters have vastly different scales. As an alternative, you could update the parameters one at the time (by specifying SAMPLING=UNIMETROPOLIS).

Tuning the Proposal Distribution

One key factor in achieving high efficiency of a Metropolis-based Markov chain is finding a good proposal distribution for each block of parameters. This process is called tuning. The tuning phase consists of a number of loops controlled by the options MINTUNE and MAXTUNE. The MINTUNE= option controls the minimum number of tuning loops and has a default value of 2. The MAXTUNE= option controls the maximum number of tuning loops and has a default value of 24. Each loop is iterated the number of times specified by the NTU= option, which has a default of 500. At the end of every loop, PROC QLIM examines the acceptance probability for each block. The acceptance probability is the percentage of NTU proposed values that have been accepted. If this probability does not fall within the acceptance tolerance range (see the following section), the proposal distribution is modified before the next tuning loop.

A good proposal distribution should resemble the actual posterior distribution of the parameters. Large sample theory states that the posterior distribution of the parameters approaches a multivariate normal distribution (see Gelman et al. 2004, Appendix B; Schervish 1995, Section 7.4). That is why a normal proposal distribution often works well in practice. The default proposal distribution in PROC QLIM is the normal distribution.

Scale Tuning

The acceptance rate is closely related to the sampling efficiency of a Metropolis chain. For a random walk Metropolis, a high acceptance rate means that most new samples occur right around the current data point. Their frequent acceptance means that the Markov chain is moving rather slowly and not exploring the parameter space fully. A low acceptance rate means that the proposed samples are often rejected; hence the chain is not moving much. An efficient Metropolis sampler has an acceptance rate that is neither too high nor too low. The scale $c$ in the proposal distribution $q(\cdot | \cdot )$ effectively controls this acceptance probability. Roberts, Gelman, and Gilks (1997) show that if both the target and proposal densities are normal, the optimal acceptance probability for the Markov chain should be around 0.45 in a one-dimension problem and should asymptotically approach 0.234 in higher-dimension problems. The corresponding optimal scale is $2.38$, which is the initial scale that is set for each block.

Because of the nature of stochastic simulations, it is impossible to fine-tune a set of variables so that the Metropolis chain has exactly the desired acceptance rate that you want. In addition, Roberts and Rosenthal (2001) empirically demonstrate that an acceptance rate between 0.15 and 0.5 is at least 80% efficient, so there is really no need to fine-tune the algorithms to reach an acceptance probability that is within a small tolerance of the optimal values. PROC QLIM works with a probability range, determined by $\textnormal{TargetAcceptance}\pm 0.075$. If the observed acceptance rate in a given tuning loop is less than the lower bound of the range, the scale is reduced; if the observed acceptance rate is greater than the upper bound of the range, the scale is increased. During the tuning phase, a scale parameter in the normal distribution is adjusted as a function of the observed acceptance rate and the target acceptance rate. PROC QLIM uses the following updating scheme: [9]

\[  c_{\mbox{new}} =\frac{ c_{\mbox{cur}} \cdot \Phi ^{-1}( p_{\mbox{opt}}/2) }{ \Phi ^{-1}( p_{ \mbox{cur}}/2) }  \]

where $c_{\mbox{cur}}$ is the current scale, $p_{\mbox{cur}}$ is the current acceptance rate, and $p_{\mbox{opt}}$ is the optimal acceptance probability.

Covariance Tuning

To tune a covariance matrix, PROC QLIM takes a weighted average of the old proposal covariance matrix and the recent observed covariance matrix, based on the number samples (as specified by the NTU= option) NTU samples in the current loop. The formula to update the covariance matrix is:

\[  \mbox{COV}_{\mbox{new}} = 0.75 ~  \mbox{COV}_{\mbox{cur}} + 0.25 ~  \mbox{COV}_{\mbox{old}} \]

There are two ways to initialize the covariance matrix:

  • The default is an identity matrix that is multiplied by the initial scale of $2.38$ and divided by the square root of the number of estimated parameters in the model. A number of tuning phases might be required before the proposal distribution is tuned to its optimal stage, because the Markov chain needs to spend time to learn about the posterior covariance structure. If the posterior variances of your parameters vary by more than a few orders of magnitude, if the variances of your parameters are much different from 1, or if the posterior correlations are high, then the proposal tuning algorithm might have difficulty forming an acceptable proposal distribution.

  • Alternatively, you can use a numerical optimization routine, such as the quasi-Newton method, to find a starting covariance matrix. The optimization is performed on the joint posterior distribution, and the covariance matrix is a quadratic approximation at the posterior mode. In some cases this is a better and more efficient way of initializing the covariance matrix. However, there are cases, such as when the number of parameters is large, where the optimization could fail to find a matrix that is positive definite. In those cases, the tuning covariance matrix is reset to the identity matrix.

A by-product of the optimization routine is that it also finds the maximum a posteriori (MAP) estimates with respect to the posterior distribution. The MAP estimates are used as the initial values of the Markov chain.

For more information, see the section INIT Statement.

Initial Values of the Markov Chains

You can assign initial values to any parameters. See the INIT statement for more details. If you use the optimization PROPCOV= option, PROC QLIM starts the tuning at the optimized values. This option overwrites the provided initial values.

[9] Roberts, Gelman, and Gilks (1997) and Roberts and Rosenthal (2001) demonstrate that the relationship between acceptance probability and scale in a random walk Metropolis scheme is $p =2\Phi \left( -\sqrt {I} c /2\right)$, where $c $ is the scale, $p$ is the acceptance rate, $\Phi $ is the CDF of a standard normal, and $I\equiv E_{f}[ ( f^{\prime } (x) /f (x) ) ^{2} ] $, $ f\left( x\right) $ is the density function of samples. This relationship determines the updating scheme, with $I$ replaced by the identity matrix to simplify calculation.