Health providers' perceptions of clinical trials: lessons from Ghana, Kenya and Burkina Faso by Angwenyi, V, Asante, K, Traoré, A, Febir, L, Tawiah, C, Kwarteng, A, Ouédraogo, A, Sirima, S, Owusu-Agyei, S, Imoukhuede, E, Webster, J, Chandramohan, D, Molyneux, C, Jones, C