To calculate a return on investment, you need to connect the communication you did with a change in audience behavior, because virtually all behaviors have a financial impact for an organization. If employees or customers or reporters do something differently, it will result in either an increase in revenues or a decrease in costs. This means you will not be able to calculate ROI based on an increase in awareness or knowledge or an improved opinion. Until knowledge and attitudes result in a behavior change, you have nothing to attach a monetary value to.
Once you quantify the behavior change, usually with the help of others in your marketing, HR or operations departments who monitor those behaviors as part of their jobs, you need to identify how much credit you can take for the resulting behavior change. It’s easiest to do this for situations where no one else was trying to influence a particular behavior so you can take 100 percent of the credit. For example, one friend of mine used communication to get employees to use a special access code before dialing long distance. The telecommunication manager gave her communication full credit for the resulting increase in the percentage of calls made using the access code, which resulted in a $20,000 a month cost savings.