The gathering of system related information about national infrastructure is key for security analysis. In the same vein, correlation involves a specific type of analysis that can be performed to identify if security-related indicators emerge from the analysis. Discuss the importance of the security principle of collection and correlation. Identify some important sources from which information needs to be collected in the context of national and critical infrastructure protection. Also, identify the four distinct analytic methods that are available for the correlation of cyber security information. Provide examples and identify challenges that are involved in collection and correlation.