Egger, M., Smith, G. D., Schneider, M., & Minder, C. (1997). 
Bias in meta-analysis detected by a simple, graphical test. 
bmj, 315(7109), 629-634.

Abstract
Funnel plots (plots of effect estimates against sample size) 
may be useful to detect bias in meta-analyses that were later 
contradicted by large trials. We examined whether a simple test 
of asymmetry of funnel plots predicts discordance of results 
when meta-analyses are compared to large trials, and we assessed 
the prevalence of bias in published meta-analyses.