saABC {abctools} R Documentation

## Summary statistic construction by semi-automatic ABC

### Description

`saABC` fits parameter estimators based on simulated data to be used as summary statistics within ABC. Fitting is by linear regression. Some simple diagnostics are provided for assistance.

### Usage

```saABC(theta, X, plot = TRUE)
```

### Arguments

 `theta` A n x d matrix or data frame of simulated parameter values. `theta[i,j]` is the ith simulated value of parameter j. `X` A n x p matrix or data frame of simulated data and/or associated transformations. `X[i,]` is a vector of the data for parameter values `theta[i,]`. A constant term should not be included. `plot` When `plot==TRUE`, a plot of parameter values against fitted values is produced for each parameter as a side-effect.

### Details

The semi-automatic ABC method of Fearnhead and Prangle (2012) is as follows:

1) Simulate parameter vectors theta_i and corresponding data sets x_i for i=1,2,...,N.

2) Use the simulations to fit an estimator of each parameter as a linear combination of f(x), where f(x) is a vector of transformations of x (including a constant term).

3) Run ABC using these simulations.

The `saABC` function automates step 2 of this process. The user must supply simulated parameter values `theta` and corresponding f(x) values `x` (n.b. excluding the constant term). The function returns weights for the linear combinations which can easily be used for step 3. In particular, fitted weights are returned as a matrix of weights for the columns of `x` and a vector of constants. The vector can usually be discarded, as it is not needed to find differences between summary statistics.

The function also returns BIC values for each parameter so that the user can judge the quality of the fits, and compare different choices of f(x). Diagnostic plots of supplied parameter values against fitted values are also optionally provided. These are useful for exploratory purposes when there are a small number of parameters, but provide less protection from overfitting than BIC values.

### Value

 `B0 ` Vector of constant terms from fitted regressions. `B ` Matrix of weights from fitted regressions. `BICs ` Vector of BIC values for each fitted regression.

Dennis Prangle

### References

Blum, M. G. B, Nunes, M. A., Prangle, D. and Sisson, S. A. (2013) A comparative review of dimension reduction methods in approximate Bayesian computation. Stat. Sci. 28, Issue 2, 189–208.

Fearnhead, P. and Prangle, D. (2012) Constructing summary statistics for approximate Bayesian computation: semi-automatic approximate Bayesian computation. J. R. Stat. Soc. B 74, Part 3, 1–28.

Nunes, M. A. and Prangle, D. (2016) abctools: an R package for tuning approximate Bayesian computation analyses. The R Journal 7, Issue 2, 189–205.

### Examples

```set.seed(1)
theta <- matrix(runif(2E3),ncol=2)
colnames(theta) <- c("Mean", "Variance")

X <- replicate(5, rnorm(1E3, theta[,1], theta[,2]))

saABC(theta, X)\$BICs
saABC(theta, cbind(X, X^2))\$BICs ##Variance parameter estimated better
```

[Package abctools version 1.1.3 Index]