Oligopoly has been among the first topics in experimental economics. Over half a century, some 150 papers have been published. Each individual paper was interested in demonstrating one effect, but in order to do so, experimenters had to specify many more parameters. Thus they have generated a huge body of evidence, untapped so far. This meta-analysis makes this evidence available. More than 100 of the papers lend themselves to calculating an index of collusion. The database behind this paper covers some 500 different settings. The experimental results may be normalized as a percentage of the span between the Walrasian and the Pareto outcomes. In the same way, results may be expressed as a percentage of the distance between the Nash and the Pareto outcomes. For each and every one of the parameters, these two indices make it possible to answer two questions: How far is the market outcome away from the competitive equilibrium? And how good is the Nash prediction? Most importantly, however, the meta-analysis sheds light on how features of the experimental setting interact with each other. Most main effects and many interaction effects are indeed statistically significant.