[AndrewPospeshil]

The number of attendees at certain events is a good place to start. If you attend, say, a parade in your city, you could send an email and ask approximately how many people were estimated to be in attendance. If the number comes from a city official/event organizer, you can assume that it'll be pretty accurate. Transparency with your data sources is important also; you should be able to prove how you got your numbers.

A survey would probably work better for initiatives that are sustained, such as mentoring a team. Because you're working closer with people, you can get a better grasp of the impact your team has. Getting interviews, personal stories, statistics, etc makes more sense for sustained programs where getting in contact with and talking to people is a lot easier.

By far the most important aspect of gathering "Chairman's Data" is being timely. It's much easier to gather and document facts about events when they've just happened. The quality of your data/documentation will be much better on the day/week of your initiative than several months/years into the future. Remember that preparing your CA submission, even if you write your essay and film your video in the build season alone, is a year-round effort.