Welchman's main work at Bletchley was "traffic analysis" of encrypted German communications. This is, first, looking for an increase in signals. Major events in warfare are signalled in advance by a huge increase in messages. Second, there are standardized parts of a message, such as message origination, message destination, time/date information, and so on. These are much easier to decipher than the actual message content. Welchman developed this approach. Today we describe this as "metadata" analysis.