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Many applications in wireless sensor networks require quality of service in terms of bidirectional reliability which consists of event reliability and query reliability. Event detection is made possible by collective information received from sensor nodes. But this may introduce congestion in the network since the information about the same event is transmitted by many nodes. This also decreases the network life time. So we go for quality based event detection by classifying the sensor nodes as essential and non-essential nodes based on contribution degree and residual energy. Essential nodes are selected so as to cover the entire sensing field. The critical events are detected by essential nodes and only the essential nodes communicate with the sink. Non essential nodes will not report the event information to the sink node. The query messages transmitted by the sink node are received by essential nodes. The required bidirectional reliability is achieved among the sink node and the essential nodes. This type of classification conserves the energy efficiently and increases the network lifetime.