## by Wally Gilks wally.gilks@maths.leeds.ac.uk

### Department of Statistics, University of Leeds, UK

Adaptive rejection sampling (ARS) is a method for efficiently sampling from any univariate probability density function which is log-concave. It is very useful in applications of Gibbs sampling, where full-conditional distributions are algebraically very messy yet often log-concave. For non-log-concave distributions, ARS can be followed by a single step of the Metropolis-Hastings algorithm; this is then adaptive rejection Metropolis sampling (ARMS).

ARS works by constructing an envelope function of the log of the target density, which is then used in rejection sampling (see, for example, Ripley, 1987). Whenever a point is rejected by ARS, the envelope is updated to correspond more closely to the true log density, thereby reducing the chance of rejecting subsequent points. Fewer ARS rejection steps implies fewer point-evaluations of the log density. Such evaluations are typically very expensive computationally in applications of Gibbs sampling.

In the original formulation of ARS, the envelope is constructed from a set of tangents to the log-density (Gilks and Wild, 1992). In a later version the envelope is constructed from chords (secants) intersecting on the log-density (Gilks, 1992). Both methods assume that the log density is concave, which is surprisingly often true even for very messy full conditional distributions encountered in Gibbs sampling. ARS is the principle sampling methodology used in the BUGS/WinBUGS project software.

I have written some FORTRAN code for adaptive rejection sampling from log-concave densities, using the 'tangent-based' method.

## Adaptive Metropolis rejection sampling (ARMS)

### Introduction

Adaptive rejection Metropolis sampling (ARMS) is a method for efficiently sampling from complicated univariate densities, such as typically occur in applications of Gibbs sampling (Gilks, Best and Tan, 1995). ARMS is a generalisation of the method of adaptive rejection sampling (ARS) (Gilks, 1992) , which was itself a development of the original method proposed by Gilks and Wild (1992). The ARMS generalisation includes a Metropolis step to accomodate non-concavity in the log density.

These notes accompany C code implementing ARMS. It should be possible to call this C code from a FORTRAN program, but I haven't tried this yet.

### Method

ARS works by constructing an envelope function of the log of the target density, which is then used in rejection sampling (see, for example, Ripley, 1987). Whenever a point is rejected by ARS, the envelope is updated to correspond more closely to the true log density, thereby reducing the chance of rejecting subsequent points. Fewer ARS rejection steps implies fewer point-evaluations of the log density. Such evaluations are typically very expensive computationally in applications of Gibbs sampling.

In the original formulation of ARS, the envelope is constructed from a set of tangents to the log-density (Gilks and Wild, 1992). In a later version the envelope is constructed from chords (secants) intersecting on the log-density (Gilks, 1992). Both methods assume that the log density is concave, which is surprisingly often true even for very messy full conditional distributions encountered in Gibbs sampling.

Occasionally however log-concavity does not obtain, typically in non-linear models, or with non-exponential- family distributions. This can be particularly frustrating when the target density is known to be very nearly log-concave. ARMS deals with this situation by performing a Metropolis step (Metropolis et al, 1953) on each point accepted at an ARS rejection step. In the Metropolis step, the new point is weighed against the previous point sampled (in applications of Gibbs sampling this is the previous value of the model parameter currently being updated by Gibbs). If the new point is rejected, the previous point is retained as the new point. The procedure is exact (in the sense of returning samples from the exact target density), regardless of the degree of convexity in the log density. However it is most efficient (rejecting fewer points at the Metropolis step) when the density is approximately log-concave. Indeed, when the density is truly log-concave, the Metropolis step will never reject.

There is, however, an overhead in using ARMS instead of ARS when the log density is truly log-concave. This is for two reasons. Firstly, each call to the ARMS function will require an additional point-evaluation of the log-density (to prime the system for Metropolis); and secondly, when log- concavity is assured, squeezing functions can be constructed which may save a function evaluation at each ARS-rejection- step. If sampling only one point from the density, (as is typically the case for Gibbs sampling) the loss in efficiency will be small. If many samples from the same density are required, the loss in efficiency would be severe. Therefore the ARMS function includes a parameter which allows you to indicate if the density is known to be log-concave. If indicated thus, the ARMS function will not implement the Metropolis step, and will exit if non-concavity is detected.

The ARMS code allows you to choose initial construction points for the envelope; (I will continue to call the ARMS rejection function an 'envelope', although for non-log-concave distributions this function is not in general an envelope any more). You can set these initial construction points anywhere you like; however, in applications of Gibbs sampling, when the distribution to be sampled is not log-concave, it is important that initial construction points are set independently of the current value of the parameter being updated. (CAUTION: the advice given in Gilks, Best and Tan, 1995, to set initial construction points based on the previous Gibbs iterate, is only valid if the target density is log-concave; a corrigendum has been submitted to Applied Statistics). In many applications, for each variable to be updated by ARMS, a fixed set of 4 or 5 initial construction points will suffice. When this would result in a large number of iterations of ARS, some exploration of the density to be sampled may assist in choosing an efficient set of initial construction points.

Both ARS and ARMS can be used to generate many points from the target density (although in applications of Gibbs sampling only one point need be sampled from each target density). As the envelope function used by both methods progressively adapts to the shape of the target density, sampling becomes progressively more efficient as more points are sampled from the target density. If the target density is log-concave, the sampled points will be independent from the exact target density. If there is non-log- concavity, the samples will only be 'from' the target density in the limit, i.e. only after many samples have been drawn, and even then they will be dependent samples. Note that this is not a concern in applications of Gibbs sampling: only one point need be sampled as the stationary distribution (of the induced Markov chain) is preserved by ARMS.

## References

Gilks, W. R. (1992) Derivative-free adaptive rejection sampling for Gibbs sampling. Bayesian Statistics 4, (eds. Bernardo, J., Berger, J., Dawid, A. P., and Smith, A. F. M.) Oxford University Press.

Gilks, W. R., Best, N. G. and Tan, K. K. C. (1995) Adaptive rejection Metropolis sampling. Applied Statistics, 44, 455-472.

Gilks, W. R. and Wild, P. (1992) Adaptive rejection sampling for Gibbs sampling. Applied Statistics 41, pp 337-348.

Metropolis, N., Rosenbluth, A. W., Rosenbluth, M. N., Teller, A. H. and Teller, E. (1953) Equations of state calculations by fast computing machines. J. Chem. Phys., 21, 1087-1092.

Ripley, B. (1987) Stochastic Simulation. New York, Wiley.