I also worked as a canvasser in 2019 and 2020 for the US census and, while we were about as thorough as you could reasonably get, the whole operation made me somewhat skeptical of official statistics in general. 2020 in particular was a bit of a disaster due to the pandemic and when the statistics were published, a bunch of mainstream news outlets published stories about certain areas experiencing "population decline" and all I could think was that those were actually the areas where the census didn't manage to count everyone.
If for example you have poor compliance with the law then the law is mostly useless (in the US you do have to update your ID in 30 days, but huge numbers of people dont).
And that doesn't count if your country has a huge undocumented population, like some places in the US do.
And how long does it take for that central registry to be informed when somebody has emigrated from the country without informing the government? Five years? Ten?
Especially anything that's self reported or whatnot. People lie. People misunderstand questions. No process is perfect.
"Just" is doing a lot of work in that sentence!
A human female can have sex once and pop out a new human 9 months later regardless of her connection to any official social systems or state apparatus. She could disappear into the woods as a hermit and produce a completely uncounted unknown new person.
To the degree that that doesn't happen, it's because a country has spent generations building a giant high trust society with good widely available medical infrastructure and a culture where almost everyone believes it is better to use that than to go it alone. Building that system requires the powerless to organize themselves and counterbalance the powerful elite who otherwise have a tendency towards despotism and corruption. That in turn requires a lot of shared culture so that the powerless feel they are all one tribe and not fractured out-groups (a reality the elites are constantly incentivized to manufacture). You need good education, mobility, safety.
An easy census is the very pinnacle of a successful society and only in a few places in the recent past has any country reached it.
Unfortunately, this extends to research studies. My mother enrolled me in the Growing Up Today Study (https://gutsweb.org/). I eventually stopped responding to that, as I couldn't see how any child (or even adult) could answer their questions on estimated food consumption remotely accurately, making the whole thing seeming dubiously ethical.
It's cited constantly in the research on ultra-processed food you see these days.
In e.g. Germany that requires a signed statement from the landlord, and the ability to receive mail at that address. If you can't receive mail at your own address, it'd be noticed and reported within at most 5 years. I actually believe it'd be the national health insurance that'd be the first to notice & report you missing, as having health insurance is mandatory (even if you continue paying them, they'd notice it once they can't send you a replacement card).
"Wow, why are the roads wearing out twice as fast as we expect?!"
frankly I don't think in any even half modern country you can go at it alone. I struggle to imagine how someone would physically manage to evade public authorities here in Germany where schooling is mandatory and any kid not in the education system would sooner or later be caught. There's barely even a place so remote authorities or other citizens would notice you and report you. You couldn't go to the doctor or anywhere really without identification or insurance.
So I think it's less of a function of trust and more simply of modernity, you're not going to escape attention for too long unless you're a trained spy or something
Where is this magical land with no homeless people?