Cooper, C;    Bou, JT;    Varley-Campbell, J;      (2019)    Evaluating the effectiveness, efficiency, cost and value of contacting study authors in a systematic review: a case study and worked example.                   BMC Medical Research Methodology , 19     , Article 45.  10.1186/s12874-019-0685-0 <https://doi.org/10.1186/s12874-019-0685-0>.       Green open access