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We were designing a feature for a system, in which a postcode search would return not the addresses within one postcode, but a list of individuals registered at that postcode.

In order for the UX design to be right, we needed to understand the average number of returns, but also the maximum.

The design, requirements team and system architects met to discuss this feature, and although no-one could answer it with data, there was anecdotal evidence to indicate that really large numbers could exist.

Do you have any examples, possibly from system design experience, where you've come across very large numbers of individuals or addresses associated with a postcode?

The question IS unanswerable, as I don't have the ability to get the postal service to do a query. That's why I'm asking for anecdotal evidence.

The UX question is about interface design. Knowing what your minimum, maximum and average results are is essential to create the right design. The reason I'm asking is that it's a question that comes up again and again in my system design so maybe others will have had the same.

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    Define 'postcode'. It appears to be like a ZIP code that some commonwealth nations use? Regardless, this question seems unanswerable short of being able to ask your nation's postal service do do a query on their own database. – DA01 Aug 11 '15 at 23:24
  • What's the UX question here? – Andrew Martin Aug 12 '15 at 4:42
  • Postcode is like a zip code for the UK – jackiemb Aug 12 '15 at 7:53
  • The question IS unanswerable, as I don't have the ability to get the postal service to do a query. That's why I'm asking for anecdotal evidence. The UX question is about interface design. Knowing what your minimum, maximum and average results are is essential to create the right design. The reason I'm asking is that it's a question that comes up again and again in my system design so maybe others will have had the same. – jackiemb Aug 12 '15 at 8:08
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    For non-UK readers: The UK Postcode system works in two parts. The "outward" code sets the sorting office that mail will be routed through and and the "inward" code sets what delivery route the address will be found on. The outward code is in two parts. A regional letter code and a number denoting a sorting office within that region. Larger offices can sometimes be further subdivided with a letter-code Thus EC1A 1BB will be found on route 1BB in the A subdivision of the 1 division of EC (East-Central London) - In most cases the number would be limited to a few hundred with few exceptions. – Andrew Martin Aug 12 '15 at 11:05
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If you're referring to the UK postcode areas, then according to https://www.mjt.me.uk/posts/falsehoods-programmers-believe-about-addresses/ Warwick University has a single postcode of CV4 7AL which refers to 6,000 students. It also gives a French postcode of 75015, referring to 230,000 people.

Forces' BFPO numbers are a single postcode, so a single aircraft carrier might refer to 5,000 or more people.

The minimum is zero since a postcode might refer to an empty or newly built building.

  • THe edit to my answer saying that there cannpt be a postcode for an uncompleted building is incorrect: Royal Mail has a database (and an interface at royalmail.com/user/login?destination=marketing-services/…) for buildings Not Yet Built. – amaca Aug 13 '15 at 20:05
  • This is just the sort of example I am looking for. So 3 pieces of anecdotal evidence now give me a maximum of 5,000 that I would realistically need to design to. – jackiemb Aug 21 '15 at 11:35
  • ps this is a theoretical exercise / fact finding mission. I am using the word 'evidence' very loosely here – jackiemb Aug 21 '15 at 11:40
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The postal code in the USA with the largest number of people in it is the one for El Paso, which has a little over 114,000 residents in a single code. There may be more in some other countries as postal codes vary from a single code for an entire city (e.g. South Africa) to a single post code for one or two streets (e.g. the Netherlands).

Relying on a given maximum number of people in a single postal code in an application strikes me as a poorly thought out design.

  • That's a really good answer re el paso. I have edited the title to specify UK postcodes. Also added a comment with a better example of how it would affect interface design – jackiemb Aug 12 '15 at 11:08
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    Yeah I agree with that last remark. There is no fixed relationship between nr of residents and nr of addresses. And even if it would, why would anyone need that knowledge for UX purposes. The nr can be anywhere between 0 and very large. Whether that is 200.000 or 2.000.000 does not matter. – Bart Gijssens Aug 12 '15 at 13:35
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Assuming that every city has a single post code and that in the extreme case every individual has its own address, we need to look at how large cities are in terms of population. The biggest city in the world is Shanghai with an estimated population of roughly 25.000.000 people. Mind that not all births may be registered correctly and take into account that the population is growing, especially in big cities. And your application must be future proof, so add a safety margin. Suggestion: assume that the largest number of private addresses is 50.000.000 and you will probably be fine.

Next, we need to look at business addresses. Every individual could have 1 or more businesses and people living outside the city can also have a business address in that city. There is probably some kind of correlation between number of people and number of businesses. Let's take a conservative, safe approach and assume that there is 1 to 1 relationship.

This would give you the nice round figure of 100.000.000 addresses. Which is nothing for any modern computer.

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Here's an example from the requirements team: 'The biggest number of addresses under one postcode that I've ever seen is 2000'

Another example from one of our SMEs: 'Many members of the armed forces will give their barracks as their address. So one barrack, with one postcode, will have hundreds of individuals associated with it'

  • Can you edit your question rather than posting an answer to your own question? – Bart Gijssens Aug 12 '15 at 10:38
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Can you specify the geographic region your application based for. Otherwise the answer for your question would be varied. I can give an example for Singapore, where postcode are actually tagged to a building, but for bigger countries it would be much bigger.

Also you can explore designs that can vary based on pre- assumed numbers, like with smaller number you could show thumbnail view, but when the result is fairly big then show list view by default giving user an option to change the view.

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The UX question is about interface design. Knowing what your minimum, maximum and average results are is essential to create the right design. The reason I'm asking is that it's a question that comes up again and again in my system design so maybe others will have had the same.

I think the real answer to your question has nothing to do with post codes and is all about number formatting. It sounds like you're interested in achieving a uniform design without having to worry about overly long number fields causing misalignment. For that end, I suggest you study 'Human Readable' number formats. This is most commonly used for file sizes – as in this question and many others on various SE sites – but is easily extended to other number fields.

The upshot is that you wind up with a display that could look something like this:

Postal Code    Addresses
C47 AL2             25
C47 AL3              1 k
C47 AL4            2.2 k
C47 AL5            100 k
           --or--
C47 AL5            0.1 M

Even non-computer/programmer types are familiar with common abbreviates for powers of 10 numbers. The only potential cause for confusion is if you're in a financial context where M is sometimes used for thousand and MM for million.

Another common application of human readable formats are the timestamp displays you can see on SE sites. This is a simple script that translates "2015-08-12 10:03:56Z" into "7 hours ago".

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