Audience data is quantitative data about the users of your website. The best-known and most easily-accessible repository of this can be found in Google Analytics (GA).
Due to their massive reach, Google can put together some pretty accurate information on who makes up your audience. They do this through a mixture of inferences and real data about users.
The inferences come from knowing what people are searching for and clicking on, while the real data is personal details they have from users who are logged into Google services (like GMail) while browsing. It's anonymised so you can't tell who the individual users are (which gives some amount of privacy).
GA’s audience data gives you some basic demographic information, such as age, gender, and location, which can help you build a picture of who your users actually are. You can then use this data to segment your audience or decide who you interview or user test with. You can build on it by surveying your users in more detail.
For the GA approach you’ll first need to turn the demographics option on, to confirm you’d like to collect this information about your users. Once it’s being collected, you then just fire up GA and head to the Audience section, where you can find the following:
You can then either summarise this data to give an overview of who your users are (with something like a user statement) or you can create personas that have come from user interviews. These personas also help you view your audience as three or four types of person, rather than just thinking of them as a single entity.
Another fruitful area to combine into your personas is the acquisition channel—does one audience tend to reach you from social media, while another from Google searches? This can shape how you speak to your audience on those different channels.
On GA, in the top right of the age, gender, and interests sections it tells you what percentage of your users it has the data for. The larger that number, the closer it is to the whole of your audience and thus reality. Small sizes on low traffic websites can mean you’re getting a skewed representations of your audience.
Don't look at too short a time period or that can also twist your data. I like to look at the last three months worth of data when assessing any GA data, so it can balance out any random fluctuations of traffic. Many sites see the make-up of their audience change at different times of the year, with big events like Christmas holidays.
It's worth checking this data every few months to see if it has shifted (once per quarter is about right). Then update the summary or outline personas you have of your audience and circulate it around your team. You should flag any notable changes.
Just because you now know more about who your users are, be careful not to make big assumptions of them, e.g. "Most of our users are young so they can figure out complex functionality". You won't know that until you get more detailed knowledge by user testing or interviewing them.
As I've focussed on here, Google Analytics (free) offers this data for your site—though you'll need to turn on the Demographics option to capture all of it. In addition, online marketing platforms like Google Adwords and Facebook Ads give you very detailed audience information about who clicks on your adverts.
It takes only an hour or two to study the data, and record a summary user statement.