**O**ur ABC survey f**or **** Statistics and Computing** (and the ABC special issue!) has been quickly revised, resubmitted, and rearXived. Here is our conclusion about some issues that remain unsolved

*(much more limited in scope than the program drafted by Halton!)*:

- the convergence results obtained so far are unpractical in that they require either the tolerance to go to zero or the sample size to go to infinity. Obtaining exact error bounds for positive tolerances and finite sample sizes would bring a strong improvement in both the implementation of the method and in the assessment of its worth.
- in particular, the choice of the tolerance is so far handled from a very empirical perspective. Recent theoretical assessments show that a balance between Monte Carlo variability and target approximation is necessary, but the right amount of balance must be reached towards a practical implementation.
- even though ABC is often presented as a converging method that approximates Bayesian inference, it can also be perceived as an inference technique
*per se*and hence analysed in its own right. Connections with indirect inference have already been drawn, however the fine asymptotics of ABC would be most useful to derive. Moreover, it could indirectly provide indications about the optimal calibration of the algorithm. - in connection with the above, the connection of ABC-based inference with other approximative methods like variational Bayes inference is so far unexplored. Comparing and interbreeding those different methods should become a research focus as well.
- the construction and selection of the summary statistics is so far highly empirical. An automated approach based on the principles of data analysis and approximate sufficiency would be much more attractive and convincing, especially in non-standard and complex settings. \item the debate about ABC-based model choice is so far inconclusive in that we cannot guarantee the validity of the approximation, while considering that a “large enough” collection of summary statistics provides an acceptable level of approximation. Evaluating the discrepancy by exploratory methods like the bootstrap would shed a much more satisfactory light on this issue.
- the method necessarily faces limitations imposed by large datasets or complex models, in that simulating pseudo-data may itself become an impossible task. Dimension-reducing techniques that would simulate directly the summary statistics will soon become necessary.