As a corollary quiz question, how many people would you need for two to have the same star sign? This is a much more tractable problem to do in ones’ head (rather than just knowing the answer, like Michael and me), but I wonder how many people would get it.

]]>Chance of being born on any day assumed to be 1/365.25

We have a remaining pool of 499 people

It is by no means certain that another person will be in the group with the same birthday but the odds ratio favour this possibility -- 1.37

If there is actually a person in the group the each person arriving has a 1/365.25 chance of matching the first persons birthday

The cumulative chance that another person with the same birthday has arrived exceeds 50 % after the 182nd person but this does not mean that you can consider the next person arriving as a 50 50 chance of sharing the first persons birthday.

Each person arriving still has just a 1/365.25 chance of sharing the first persons birthday.

I don’t think your odds will ever be 50:50 unless you are tipped off that there is a person in the group which shares the birthday and even then will not be 50:50 until they haven’t arrived until after the 497th person.

your question demonstrates the difficulty of conceptualizing low risk possibilities applied to actual people and groups, i couldnt pull any more complex math from distant stats and high school to help me

]]>I work in a rural nth qld hospital setting

I agree that we need to be circumspect about CTing suspected renal colic patients.

However, in my mind the size and position of the stone are very useful pieces of information which USS cannot often answer.

If a patient presents for the first time with typical renal colic pain, haematuria and is found to have a 10mm stone, hydroureter, hydronephrosis on CT then that will effect my disposition plans leaning towards early involvement of a urologist, whereas a 2mm VUJ stone can safely be sent home with analgaesia and a strainer.

It is true that developing self sufficiency in renal/ ureteric scanning can help in making the diagnosis out of hours, and help exclude other uglier diagnoses but sending patients with large calculi home generally leads to unseemly representations at odd hours.

I often would use USS to diagnose renal calculi/ hydronephrosis/ hydroureter but then in patients presenting for the first time or the first time in years would follow that with CT to characterize the calculus. The old method of plain film KUB to document movement of the calculus over the next days to weeks is sometimes is helpful when the stone is radioopaque.

The NEJM article appears to refute my clinical beliefs about renal calculi but i have only been able to read the abstract. Size does matter with calculi doesnt it?

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