FAA Statistics - Airport version:

Have you ever looked up an airport online and seen the statistics the FAA compiles for an airport - including number of aircraft on airport, number of operations (take-offs and landings) per day, and type of traffic? How do they get those numbers for aircraft without control towers?

They don't, actually. There is no Federal big brother hiding in the weeds off-airport and writing your tail number down - they use statistics, instead. Interestingly enough, some states have used acoustic counters, machine counts, surveys, and operations logs, the FAA relies on the surveys filled in by pilots, the mandatory reporting by flight schools, and mostly, statistics.

Yeah, 'lies, damn lies, and statistics.' Including the lumping of towered and untowered airports together. See: report here.

Some interesting items do come of reading the report the statisticians put out to defend their accuracy. For example, "Other things equal, it seems that the number of operations grows as the number of based aircraft increases, but at an increasingly slower rate. This “slowdown” is governed by the negative coefficient on the square of based aircraft."

And this footnote: The negative parameter estimate for the airport’s percentage of based aircraft among all based aircraft at airports within 100 miles is somewhat counterintuitive. It might be thought that an airport with more of an area’s based aircraft would have more operations, other things equal, but this seems to be true only relatively. What seems to be more important is that if a single airport is dominant (in terms of based aircraft) within the surrounding 100 miles, then the area itself has relatively low levels of GA activity, even if that dominant airport has the bulk of it.

and on massaging the equations to try to model reality:

In Equation 17, operations per based aircraft are reduced (i.e., have negative coefficients) for airports with greater than 100 based aircraft, for those in the Western or Alaska regions, for those that are “regionally prominent” in the sense that they have a large proportion of all based aircraft at nearby airports, and for those with relatively higher populations within 50 miles. Operations per based aircraft are modestly increased (i.e., have positive coefficients) for those airports with relatively higher populations within 100 miles.

So, next time you look up an airport's activity, just remember that if it doesn't have a tower, it's a government-created statistically modeled number supposed to reflect reality. Which is to say, a good guess.