What is the difference between deep packet inspection and stateful inspection?

Deep packet inspection (DPI) is a form of computer network packet filtering that examines the data part (and possibly also the header) of a packet as it passes an inspection point, searching for protocol non-compliance, viruses, spam, intrusions, or defined criteria to decide whether the packet may pass or if it needs to be routed to a different destination, or, for the purpose of collecting statistical information. It is a form of filtering employed at the security layer level of the OSI model. Stateful inspection, on the other hand, is a firewall technology that monitors the state of active connections and determines which network packets to allow through the firewall. Stateful inspection has largely replaced older technologies that were static and examined packets in isolation. Therefore, DPI is considered more secure because it examines the contents of the packets at Layer 7 (the application layer), while stateful inspection typically works up to Layer 4 (the transport layer).