Principled Comparisons for End-to-End Speech Recognition: Attention vs Hybrid at the 1000-hour Scale

End-to-End speech recognition has become the center of attention for speech recognition research, but Hybrid Hidden Markov Model Deep Neural Network (HMM/DNN) -systems remain a competitive approach in terms of performance. End-to-End models may be better at very large data scales, and HMM / DNN-systems may have an advantage in low-resource scenarios, but the thousand-hour scale is particularly interesting for comparisons. At that scale experiments have not been able to conclusively demonstrate which approach is best, or if the heterogeneous approaches yield similar results. In this work, we work towards answering that question for Attention-based Encoder-Decoder models compared with HMM / DNN-systems. We present two simple experimental design principles, and how to build systems adhering to those principles. We demonstrate how those principles remove confounding variables related to both data, and neural architecture and training. We apply the principles in a set of experiments on three diverse thousand-hour-scale tasks. In our experiments, the HMM / DNN-systems yield equal or better results in almost all cases.