We're not big fans of Google Anlytics or serivces like Hubspot that hoover up as much information they can grab. They're great if you're running a shop and need to get as much data as possible to help sell your widgets, but for most of our clients it's overkill and honestly feels a bit grubby.
So, we persuaded one of the sites we look after to move away from Google Analytics and switch to a more privacy friendly analytics service.
In order to check any variations between the number of visitors that the different analytics packages were recording, we ran both on the site for a couple of months. The site was also behind a Cloudflare instance to make use of their firewall and content delivery network.
Since Cloudflare also collects visitor statistics this meant that we had three different measurements of visitor numbers. When we came to compare them there were some quite startling differences.
The following screenshot shows the different visitor tallies for the first 30 days of 2023:
about 37K visitors according to Google,
42K for Fathom, and
80K for Cloudflare
Our guess is that Google is both fussiest about tying requests to real people (cos hey that's what they're after), but also most likely to be blocked by ad blockers and privacy tools.
Fathom is probably less likely to be blocked by privacy tools; we use a bespoke domain which hopefully isn't blocked that often.
Cloudflare isn't is going to be blocked by ad blockers because it's gathering stats at sever level, but maybe is more lenient about what constitures a 'visitor'. We don't have details about that but it is a big jump from Google.
The take away from this is that stats packages like these really aren't much cop at telling you absolute visitor numbers. What they can do though is show you trends about how your site is growing (or not) over time.