SARCS strong-lensing galaxy groups - II. Mass-concentration relation and strong-lensing bias G. Foëx, V. Motta, E. Jullo, M. Limousin and T. Verdugo A&A, 572 (2014) A19 Published online: 20 November 2014 DOI: 10.1051/0004-6361/201424706