The emission of an electron pair upon single-photon absorption requires a finite electron-electron interaction. Therefore, double photoemission is a particularly sensitive tool to study the electron correlation in matter. This is supported by a recent theoretical work which predicts that the pair intensity is a direct reflection of the correlation strength. In order to explore the validity of this statement, we performed a study on a variety of materials. Among them are noble metals, transition metals, and insulators. The latter include transition metal oxides such as CoO and NiO which are also termed highly correlated. We find an increased pair emission rate of NiO and CoO compared to the metals which reach a factor of 10.We also discovered that an increase of the coincidence intensity is accompanied by an increase in the singles count rate. This demonstrates that the electron pair emission is an efficient process at surfaces contributing up to 15% to the single-electron emission in double photoemission. We performed also electron pair emission studies upon primary electron impact and find similar intensity relations.