The ruling body has decided to combine the yearly census with a head-tax, how can i avoid rampant tax avoidance and wrong data?

Meticulous Clerical records from the Clergy and the Ruling style of Vlad the Impaler

Okay - Parody and mentioning 40K - I'm absolutely going to be referencing the Ecclesiarchy - now for those not familiar - in WH40, there is the Human faction that maintains control via a religious theocracy (there are other elements) - but think the Roman Catholic Church at the height of it's power and influence... Then multiple that by a factor of 10 (or 40...) - and you get the idea.

So - each area has a local Church - which officiates all Marriages, Births and Deaths - and whereas in the WH40K lore - this is a sprawling mess owing to size and it is a parody of the Civil Service with it's inefficiency - we are going the other way - the Clergy, for all their faults are absolutely meticulous in their record keeping.

Consequently the state already 'knows' how many people there are...

Why bother with the head tax then?

Well, one of the most infamous individuals from History is one Vlad Tepes - Aka Vlad Dracul Aka Vlad the Impaler - the man that inspired Count Dracula.

There are many stories about him - but one theme does seem to be very consistent - he was all about honesty and Loyalty. If you were honest, you'd be fine. If not, you'd be brutally tortured and executed usually by impalement (hence the name).

There are stories about him leaving a golden cup in the middle of a village for 24 hours, coming back to it a day later and it still there, paying a merchant for the missing costs of goods, but adding a single extra gold coin to test the Merchants' honesty (the Merchant passed) etc.

Whilst most of these stories are certainly apocryphal - the history books says that Vlad was (even by the cruel standards of the day) quite vicious - but ironically, his brutal method of ruling helped protect Wallachia - and so today he is beloved in Romania. History is complicated.

Anyways.

The Tax is deliberately small - a single coin because it is not about the monetary value, it is about the principle in being honest with your dealings at all times.

Pay the correct amount, or risk being brutally tortured and executed - it reinforces that a small, honest sacrifice is better than the alternative.

I think this gives both an internally consistent answer, plays into the Parody of certain settings and has a few historical (or more likely apocryphal) callbacks.

Do it like the Romans did

The Romans were very aware of the problem of tax dodging, especially in the remoter regions of the empire. So they came up with a solution: instead of sending officials to distant regions where they don't know the people, the language or the culture, they hired local residents. Good luck explaining that little Timmy died of plague to Bob down the street who saw him walking to school yesterday.

It gets better. Rather than tax collectors being paid a salary, the right to collect taxes in an area was auctioned off to the highest bidder, the government gets the amount bid, and anything they collect above that amount is profit for the collector (and if they overbid, they lose, not the government). Your expected tax is pretty easy for the locals to estimate (it's just the number of people in the town), so if the bidding process is at least somewhat competitive, the bids will be pretty close to the population. So your government can take the sum of the bids and add, say, 10% to get a pretty good initial estimate of the population while waiting for the results of the census. Your tax collector has both the ability (they can't hide from him indefinitely) and the motive to collect from as many people as possible. At the end of the census he has no reason not to turn in an accurate head count since the difference is going into his pocket anyway.

Having organized religion, in which the church maintains careful records of births, deaths and marriages will help, but isn't absolutely essential. Yes, your tax collector will be the most hated guy in the community. Given your description of the scenario, that may be a feature, not a bug.

Well, let's just say that the census bureau and tax office is also the electoral commission and (only permitted) estate agent.

So, when the census and head-tax officers come around, they take names and coins, but they also register voters... and every registered citizen can vote, right down to newborns, up to the ancient and demented, the parents voting for the kids who aren't yet old enough, and the kids voting for their senile seniors. If you dodge the census/tax, you dodge the vote too.

Of course, some people would rather dodge the census, head tax and the right/duty to vote, and that's where the national estate agency comes in.

If a house is empty when the government agents come around, it's obviously abandoned, so it goes up for sale, the proceeds going to the government. If it's a lot bigger than the occupants present need, it also goes up for sale, since the occupants are obviously empty-nesters or not as fertile as they had anticipated, and need help downsizing to one of the empty smaller houses. On the other hand, a household with too many people in it will be helped to move to a bigger residence. "No need to thank us, it's all part of the service your government provides."

So, with the incentives of A): being able to vote, and B): keeping your house, there's plenty of reasons for you to tell the government about all the members of your family.

But wait! There's more!

What if you're rich, with a big house and a small family, and a desire to have your votes counted? You could pay poor people a coin each to come to your oversized mansion and be counted as part of the von Richie family when the government agents come around so that you don't look like you have an oversized house, and you get some extra votes. The poor people you hired now have enough money to pay their own head tax as part of their own Nodolla or Povvo families so that they don't lose their own house and votes.

If you're rich, but don't (yet) have a big (enough) house, you can pay to pad your family so that you'll get a bigger house... at the expense of someone who has been under-reporting the size of their own family.

So what if you're fudging the census and the electoral roll? It's the government's fault for coming up with such a ridiculous system in the first place.

However, the government is happy with this: They get to point to their (supposedly) burgeoning population and (genuinely) large tax revenues... and with more people on the census than in reality, there's less pressure on the government infrastructure budget, and the unincentivised bureaucrats of the national estate agency have less pressure too. It's a win-win all around.

Good answers already, but there is a better way than a tax. Give an incentive for people to register in the census, no need to even tell them they're being counted. Some countries do it every few years.

They take some of the tax money and give a small payment to everyone. So my country is going to go door to door and give everyone 150 to help with school fees and uniforms soon out of kindness and love (school started a month ago, but it sounds great). So everyone is excited and not questioning any underlying motivations or what will be done with the records. Then probably in a short while we'll all have digital Id's they've been telling us we need.

About 5 years ago it was 50 each person for some other reason I don't recall. But obviously it's an excellent way of getting everyones details and location for govt use.