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Comment by cyberferret | Hacker News Reader

In a few stores here in Australia, I am asked for my post code (or country of residence if no post code) by the cashier when ringing up a sale. Predominantly at tourist/tour related points of sale, but I've also had it an electronics and white goods stores.

No idea if the operator is also recording gender and perceived age group etc., but I do know that on most occasions, you can opt not to answer the post code question.

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When you go to those weekend open inspections in Sydney, you are almost guaranteed to be asked for your postcode. They record that as well.

I actually did some experiments - for different properties in roughly the same area/price range, I told different real estate agents different postcodes. It is beyond reasonable doubt that the code you tell them play a huge role on how they rank you as potential buyers. When you tell them a random north shore post code, you are guaranteed to receive a nice & friendly follow up on the coming Monday, however if you tell them that you live in the west (when mostly inspecting north shore properties), they would smile and immediately end the whole conversation.

The sample size here is ~50, which I believe is big enough to draw some reasonable conclusions.

If you give them a mobile phone number (which they all ask for) they will simply use that to track you far more closely than than can with a postcode

How? As far as I know a commercial company can't get your location or other personal information just by knowing your mobile phone number?

You are fooling yourself here. They probably have a whole data cake already, they want the mobile number as a cherry topping on this cake.

You know about the anonymized and aggregated data that one can buy ? Well it can be easily anonymized and deaggregated.

The good part is that you don't even have to do it yourself and go directly to data brokers that have done the hard work for you.

Coming from the standpoint of a very curious independent researcher, I'm curious what I might query/search for to learn more about these data brokers.

Not at all out of anything that might be categorized as malice, just to add this datapoint to my mental map.

Buying data lists. You usee your mobile number to get your cinema ticket - or whatever - the cinema sells that data. Do you get package alerts when you have a parcel due, now your phone number is joined to that address, plus presumably the credit card companies sell their data (?).

Companies amalgamate that data, then sell lookups of varying degrees.

Screwfix in the UK gather a lot of personal data as part of their sales process, they're the least covert about data gathering I've seen.

I'm not sure if it's true, but a friend once told me the reason a store asked for your zip code was to see if they had a large audience coming from a certain area. This let them know other locations to possibly open other stores.

As far back as 2000, my partner was asked for her zip code as we made a purchase and I curtly replied "no comment," to the surprise of everyone there. She walked for her phone number, which really irritated me because this was a $5 retail purchase of some type), and I got more curt when I said, "You don't need that!"

I've been annoyed by this stuff long before most people were ready to consider privacy concerns anything more than paranoia.

I like to give fake numbers in these situations. The way I see it, intentionally supplying bogus data is one of the only ways we have left to fight the machines and their algorithms!

I like to give obviously fake numbers. Like 12345 for a zip code or 212-555-1234 for a phone number. Most people don't care enough to have a reaction, now and then you get a laugh, and rarely you'll get someone who calls it out as bogus. My standard retort is somewhere between "Are you saying I don't know my own phone number?" and "Are you calling a liar?!" depending on how surly the response.

I was in an albertsons back when they wanted a number in tahoe and went in the 2nd time... cashier remembered me and said "what's that number again... something something something 5309eynine..." and was dancing a bit... it took me a second then I said "what's the area code here?" he glady gave it to me, so for about 12 years I just did $CURRENT_ZIP-867-5309.

My safeway card is in someone else's name... one day they had to pull it for some reason and I got a "Have a nice day.... Mr.... Soprano." and a big smile.

My father had memorized a fake Social Security Number that had come as a sample card in a wallet he got in the 1950s. When anybody except the government asked for his SSN and who wouldn't relent on his pushback, he would give them that number.

It wasn't 078-05-1120?

https://www.ssa.gov/history/ssn/misused.html

Wow, nice! I don't know if he was one of the 12 in 1977, but he would have been if this is the number he used. Woolworth's totally makes sense. If he were alive, he would poop purple Twinkies at that story. Thanks!

How does that work for him for things that do credit checks?

He's been dead for 10 years, but he didn't use it for those. As I remember it, in the 80s-90s it was more common for SSNs to be requested for normal consumer things.

!!!!!!! Can't wait for those numbers to leak out!

I gotta say, it's possible he never actually used it in my lifetime, but he could sure tell that story and rattle off the digits at a moment's prompting.

Some countries have law making it illegal to give bogus information about you, with hefty fines and jail time.

There's a bit of plausible deniability if you give slightly bogus info, like transposing numbers. You could assert that there was a typo on the company's part.

Really? I say "Nope" constantly and they're like "no problem" or "it's way easier to look up refunds that way" which is true depending on the store.

If you purchased with a card, they can usually look up a transaction based on that.

> "it's way easier to look up refunds that way"

"Thanks good to know, from now on I'll go buy at a business where this artificial limitation does not exist."

The GM of the big box that I worked at 25 years ago said that the zip code thing was to measure the "destinationness" of the store.

In our case, the average big ticket buyer travelled an average of 30 miles, which was awesome in that it made the end caps more valuable.

Sometimes the credit card terminal will ask especially at gas pumps for my zip code as an anti fraud measure, and will reject the transaction of you enter the wrong number. If a cashier is asking I always use 90210.

I guess a lot of people not from the US will use 90210 when prompted for a US postal code. I can't even remember what the show was about (except that it was set in Beverly Hills, obviously), but the number stuck.

The show was called 'Beverly Hills 90210'

Just a generic teen drama

I had same issue when visiting the US. I tried some fake numbers but it wouldn't accept it (my actual postal code has letters, so that wasn't possible), so I just ended up paying by cash.

Did you try your postal code minus the letters? Afaik[1] usually only the numbers in your address are verified anyway.

[1]: https://en.wikipedia.org/wiki/Address_Verification_System

My UK postal code only has two numeric digits, whereas I seem to recall the US gas pumps require five digits. Just means you can't pay at the pump.

Post codes in Australia are just 4 numbers, so when buying subway tickets in NYC, I just put in 10000 or something (I believe that's close enough to the local code?).

I've heard (unsure if true) it was to reduce entropy.

IE they keep the last 4 digits of your credit card, combined with a post code you have a unique id.

Not necessary, most point of sale systems can provide a unique hashed or tokenized version of the account number for analytics and identification purposes.

By cross-referencing your name (from your credit/debit card) with your zip/post-code, stores are able to determine specifically who you are with greater probability than without the zip/post-code.

With near 100% probability actually. There was a detailed report on that few years ago.

Googling for "why stores ask for the zip code" brings up a lot of press post, e.g. https://www.forbes.com/sites/adamtanner/2013/06/19/theres-a-...

This is the correct answer.

I always give the post code of the shop, if I know it, or one nearby that I do know.

Wait, why do you know the post codes of the places you shop?

Wait, you don't? :-)

Seriously, I mostly shop in the same neighbourhoods. And when not, there's often something on the counter with their address...or I can give a mate's address and let him get the junk mail....

Isn't it a credit card security feature? That's what it's for at gas stations.

Correction, that what they pretend it is for at gas stations.

99,99% of the time when asked a piece of personal data it is for cross-referencing or other privacy invading purpose.

I think they mean if a salesperson asks you for it, rather than when you put it in for a credit transaction.

Of course you can refuse, but you can also just give them a false one.

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