In autonomous driving, object detection is an essential task to perceive the environment by localizing and classifying objects. Most object detection algorithms rely on deep learning for their superior performance. However, their black box nature makes it challenging to ensure safety. In this paper, we propose a first-of-its-kind methodology for statistical analysis of the influence of various factors related to the objects to detect or the environment on the detection performance of both LiDAR- and camera-based 3D object detectors. We perform a univariate analysis between each of the factors and the detection error in order to compare the strength of influence. To better identify potential sources of detection errors, we also analyze the performance in dependency of the influencing factors and examine the interdependencies between the different influencing factors. Recognizing the factors that influence detection performance helps identify robustness issues in the trained object detector and supports the safety approval of object detection systems.