Normal market sizing approach was taken. You would first size the population of US who travel by flight in a year within US. Then segment that number into the number of people who could travel to Illinois. Now even within Illinois, you would further segment the overall number of airports and try coming up with a ball park number. Onto top of this, you would even consider the number of people who fly into Chicago from international destinations. Finally then you need to estimate the number of people per flight and then calculate your final number.