AZDuffman
AZDuffman
Joined: Nov 2, 2009
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March 4th, 2021 at 4:12:45 PM permalink
I was dealing with a large USA bank this week. Instead of an account number they asked for the person's last 4. To my surprise this identified the person with no other info needed! The place has millions of customers.

So, what are the odds if you call a bank and give the last 4 of your SS number that no other customer has it with:

100,000 customers
500,000 customers
1,000,000 customers
2,000,000 customers
4,000,000 customers

And what is the way to figure this? Stats class was a lifetime ago and I do not remember this kind of problem. I imagine it is done like the odds of 2 people having the same birthday in a group of 30 where you figure the odds of NOT having it and reverse it?
All animals are equal, but some are more equal than others
gamerfreak
gamerfreak
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March 4th, 2021 at 4:34:45 PM permalink
Quote: AZDuffman

I was dealing with a large USA bank this week. Instead of an account number they asked for the person's last 4. To my surprise this identified the person with no other info needed! The place has millions of customers.


WHAT?!

I donít even think itís unlikely for multiple customers to have the same last 4.

https://www.ssa.gov/history/ssn/geocard.html
AZDuffman
AZDuffman
Joined: Nov 2, 2009
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March 4th, 2021 at 4:39:20 PM permalink
Quote: gamerfreak

WHAT?!

I donít even think itís unlikely for multiple customers to have the same last 4.

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



That is my point, only 10,000 possible combinations assuming none are reserved (0000?) So many should have the same, but the one I called with (my job to call) they had to be the only one as Caller ID would not equate my number with one.
All animals are equal, but some are more equal than others
darkoz
darkoz
Joined: Dec 22, 2009
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March 4th, 2021 at 4:54:29 PM permalink
Quote: AZDuffman

I was dealing with a large USA bank this week. Instead of an account number they asked for the person's last 4. To my surprise this identified the person with no other info needed! The place has millions of customers.

So, what are the odds if you call a bank and give the last 4 of your SS number that no other customer has it with:

100,000 customers
500,000 customers
1,000,000 customers
2,000,000 customers
4,000,000 customers

And what is the way to figure this? Stats class was a lifetime ago and I do not remember this kind of problem. I imagine it is done like the odds of 2 people having the same birthday in a group of 30 where you figure the odds of NOT having it and reverse it?



They asked for NOTHING else?

Not for person name or birthday?

You just called and they said what is your last four digits of SSN and they suddenly knew who you were?

And we're you calling from your cell phone? This is important as many places will have cell info.

So you call from cell, it is recognized as belonging to you personally, they just ask your last four SSN to make sure it's not someone else using your phone
For Whom the bus tolls; The bus tolls for thee
heatmap
heatmap 
Joined: Feb 12, 2018
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March 4th, 2021 at 5:06:41 PM permalink
i can get the last 4 digits of your credit card just by following you to a gas station and using the one in whatever times you forget the receipt

i can get your name by going through your trash

etc etc
AZDuffman
AZDuffman
Joined: Nov 2, 2009
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March 4th, 2021 at 5:19:10 PM permalink
Quote: darkoz

They asked for NOTHING else?

Not for person name or birthday?

You just called and they said what is your last four digits of SSN and they suddenly knew who you were?

And we're you calling from your cell phone? This is important as many places will have cell info.

So you call from cell, it is recognized as belonging to you personally, they just ask your last four SSN to make sure it's not someone else using your phone



No, I explained above it was not mine, it was a client. So my phone would not match the number they had on file. Yes, I punched in the last 4 and they found the account on that alone. With a couple million customers I want to know how long the odds are.
All animals are equal, but some are more equal than others
unJon
unJon
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March 4th, 2021 at 5:27:41 PM permalink
Quote: AZDuffman

No, I explained above it was not mine, it was a client. So my phone would not match the number they had on file. Yes, I punched in the last 4 and they found the account on that alone. With a couple million customers I want to know how long the odds are.



Could be if your last four returned multiple possibilities then they would have asked a follow-up.

You are correct that this is just the birthday match formula.
The race is not always to the swift, nor the battle to the strong; but that is the way to bet.
gamerfreak
gamerfreak
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March 4th, 2021 at 5:30:18 PM permalink
Quote: unJon

Could be if your last four returned multiple possibilities then they would have asked a follow-up.

You are correct that this is just the birthday match formula.


This makes more sense.

Not sure why but I thought AZD was working at/with this bank in some professional capacity and knew they were only referencing account by last-4.
AZDuffman
AZDuffman
Joined: Nov 2, 2009
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March 4th, 2021 at 5:43:06 PM permalink
Quote: unJon

Could be if your last four returned multiple possibilities then they would have asked a follow-up.

You are correct that this is just the birthday match formula.



Of course they would ask more, I have had them do so. This time there were none so means only 1 person had that last 4.
All animals are equal, but some are more equal than others
unJon
unJon
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March 4th, 2021 at 6:17:05 PM permalink
Quote: AZDuffman

Of course they would ask more, I have had them do so. This time there were none so means only 1 person had that last 4.



Well thatís a different a question and not the birthday problem. Itís the chance this particular person doesnít have a last four as any of the other X people.

Chance = 1 - (9,999 / 10,000 )^X
The race is not always to the swift, nor the battle to the strong; but that is the way to bet.

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