I consider any of these types of studies to have faulty logic.
Humans have only been creating electromagnetic signals for about a century. None of them make it to Alpha Centauri, four light-years away. There isn't a radio transmitter with the power.
Furthermore, as we enter an age in which we live largely in virtual worlds of our own design, strong radio emissions (radio, TV) are decreasing. Sure, there's plenty of wifi, but we all know that signal won't make it more than a couple of blocks, let alone four light-years.
Perhaps it's that I'm in the field, but it seems to me that we're ultimately headed for a world in which human consciousnesses are housed in something we would not today recognize as a machine.
If other civilizations followed our same path (i.e. trapped by the speed of light within our own solar system), then their radio emissions would never be detected. They'd also be fairly short-lived -- a couple of centuries at best.
Once you're a "download" (for lack of a better word) and living your life entirely in virtual worlds that only interface with the real world for power and maintenance, why would you broadcast anything?
In short: these types of approaches assume that civilizations emit greater energy and detectable emissions the older the civilization is. I suspect the reverse is true: the older the civilization, the less it emits.