Associations with antibiotic prescribing for acute exacerbation of COPD in primary care: secondary analysis of a randomised controlled trial by Gillespie, D, Butler, CC, Bates, J, Hood, K, Melbye, H, Phillips, R, Stanton, H, Alam, MF, Cals, JW, Cochrane, A, Kirby, N, Llor, C, Lowe, R, Naik, G, Riga, E, Sewell, B, Thomas-Jones, E, White, P, Francis, NA