1. Co-amoxiclav in recurrent acute otitis media: placebo controlled study;Appelman, C.L.M.; Claessen, J.Q.P.J.; Touw-Otten, F.W.M.M.; Hordijk, G.H.; de Melker, R.A.;BMJ,1991
2. Acute suppurative otitis media;Glasscock, M.E.; Shambaugh, G.E.,1990
3. Nonparametric statistics for the behavioral sciences;Siegel, S.,1956
4. Case-control studies;Schlesselman, J.,1982