In-network data gathering and data fusion are essential for the efficient operation of wireless sensor networks. While most existing data gathering routing protocols addressed the issue of energy efficiency, few of them, however, have considered the quality of the implied data aggregation process. In this work, an information model for sensed data is first formulated. A new metric for evaluating data aggregation process, Data Aggregation Quality (DAQ), is formally derived. DAQ does not assume any prior knowledge on values or on statistical distributions of sensing data, and may be applied to most data gathering protocols. Next, two new protocols are proposed: the Enhanced LEACH and the Clustered PEGASIS, enhanced from two major existing protocols: the cluster-based LEACH and the chain-based PEGASIS. By carefully accounting for listening energy, energy efficiency of all four protocols is evaluated. In addition, DAQ is applied to evaluate their data aggregation process. It is found that, while chain-based protocols are more energy efficient than cluster-based protocols, they however suffer from poor data aggregation quality. DAQ may be readily applied to most of continuous data gathering protocols; it is therefore significant to future development of sensor network protocols.