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Eggstrodinary!

keefus's picture

I bought half a dozen eggs a few days ago. So far I've used 3, and every one had a double yolk. Any odds on them all being double?

I'd be eggstatic if they were.

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Funny you should ask

Dealt with pretty well here:

Take a wonderfully trivial example: last month it was reported that a shopper had bought a box of six eggs, and all had double-yolks — a “one-in-a-trillion chance”. This calculation was explained on Today by a man from the Egg Council who said that, as one in 1,000 eggs were double-yolkers, the chance of all six being double-yolkers was one in 1,000 x 1,000 x 1,000 x 1,000 x 1,000 x 1,000.

This aroused my suspicion. To begin with, this is not a trillion and, if this were the true chance, we would expect to wait 500 million years before this event occurred. So what has gone wrong with this model? It turns out that the egg-world may not be so simple. Double-yolkers are far more common in certain flocks, so there should be uncertainty about the “one in 1,000” parameter. Eggs in a box tend to come from the same source so once one double-yolker is found the chances increase that the rest will match. As a result there is uncertainty about the structure of the model too.

Acknowledgement of parameter and structural uncertainty has become common in climate and other models. But there is a further level of uncertainty: of unforeseen surprises, Black Swans and Rumsfeldian unknown unknowns. There should always be a suspicion that there’s more going on than we can express in mathematics. Indeed, at Waitrose I bought a box marked “double-yolked eggs” for £2.49. Certainly not a one-in-a-trillion chance: double-yolked eggs can be common and can be detected, selected and packed at will.

Via http://understandinguncertainty.org/blog/6?page=1

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SpaceBoy | 25 July 2010 - 1:20pm

Yolky dolky

Richard Dawkins deals with this kind of thing - our perception of coincidences - and more in his best book, "Unweaving the Rainbow":
http://www.amazon.co.uk/Unweaving-Rainbow-Science-Delusion-Appetite/dp/0...

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Nick White | 25 July 2010 - 2:45pm

The thing is though that

if the stats of putting each egg in the box *were* independent, and if double yolks *were* rare (e.g. 1 in 1000), then the combined odds of 6 in a row would indeed be pretty unusual. So it's not really about his perception of coincidences. It's about how likely a box actually is to contain 6 double yolked eggs.

Keefus' question is sensible, imo. Implicit in it is the question of whether seeing 3 in a row has now made it more or less likely that he'll see a full 6. In "O Level" textbook stats, based on completely independent events, seeing 3 (or even 5) makes no difference to the overall final answer.

In the human world we live in though, other factors are relevant. They don't mean stats is bogus-they mean that it has to be used extremely carefully (c.f. the blog I quoted).

For all the human reasons cited in Prof Spiegelalter's blog I'd agree wqith him that seeing 3 in a row *in this context* may well suggest that the chances of a full box are quite high. My street-smart FPO however reckons that the chances are actually lower than random, because most of the double yolkers have already been nabbed for the special boxes-they should weigh more ? Perhaps K will find he has a box of 5. It was meant to be a box of 6 and failed it's final QC check ;-)

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SpaceBoy | 25 July 2010 - 4:32pm

Woah, Nick...

Too much maths - I want out!
(*gives blank Father Dougal expression*)

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Nick White | 25 July 2010 - 4:42pm

Fair enough

but Spiegelhalter tells it well-it is after all his job. And unlike Dawkins he is a statistician ...

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SpaceBoy | 25 July 2010 - 4:45pm

The yolk delusion

I don't think Spiegelhalter and Dawkins are on opposite ends of any argument here.
Dawkins just gives some examples in life where people think a massive coincidence has occurred (and often attribute it to supernatural forces) when in fact, if all the stats are considered, it's to be expected that such things would sometimes happen, and not as rarely as might be naturally thought.
He doesn't use the double yolk eggs example; one example he does use is the likelihood of two people in a room sharing the same birthday, which is more likely than most people would think. His main point is against psychics who play the odds to exploit people.

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Nick White | 25 July 2010 - 5:07pm

OK

I'm sure they aren't ideological opposites. But in the stats context they are actually making different points.

In Dawkins' example the likelihood of 2 people in a room sharing a birthday is about (1/365)*(1/365), assuming independence and that any bday is as likely as any other. In round numbers that's v roughly 1/400 times 1/400, i.e 1 in 160 000, so a small chance but not tiny, in a room of a few hundred people for example.

The odds of 6 eggs in a row being double yolkers, *if* we can assume that any individual one has prob of 1 in a thousand, and *if* they are independent events, is much much smaller (1/1000) to the 6th power. Spiegelhalter's point is that the reason we still actually see them quite often is that both the model we used, of independence, is not at all applicable, *and* the 1 in 1000 is also a dodgy number-whereas it's not so obviously a bad assumption for birthdays, is it ?

Anyway, I'll shut up now.

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SpaceBoy | 25 July 2010 - 5:43pm

Dawkins & Spiegelhalter

I'm sure they could discuss it amicably over a nice omelette.

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Nick White | 25 July 2010 - 5:53pm

Indeed

cheers

N

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SpaceBoy | 25 July 2010 - 6:01pm

Not wishing to fuel the fire

But the chances of two people sharing a birthday are nothing like 1 in 160000.

If you were to pick two people at random, the chances of today being both their birthdays is around the 1 in 160000 you mention.

BUT the chances of them sharing a birthday are nothing like that - it's 1 in 365 (excluding the possibility of February 29). Basically, we KNOW when person #1's birthday is, so the probability of that is 1. Then, the independent event that is person #2's birthday? Well, the probability of that happening on the same day is 1 in 365. Thus 1 * 1/365 = 1/365.

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Joe R | 25 July 2010 - 6:15pm

Fine

I was assuming we didn't already know it was person no 1's bday, but if we do e.g. the 2nd person is at person no 1's party then fine, you are right.

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SpaceBoy | 25 July 2010 - 6:25pm

Also

It depends on the number of people in the room. Dawkins was talking about the pigeonhole principle, whereby the likelihood of two people sharing the same birthday in a room containing 57 people is 99%. With only 23 people in the room, the likelihood is still 50%.

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Fraser Lewry | 25 July 2010 - 6:54pm

I'm a dog of little brain but...

he suggests, 'if this were the true chance, we would expect to wait 500 million years before this event occurred.'... but why couldn't today be the day it happened, then for it not to happen for another 500 million years or thereabouts? Or am I a meringue?

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badartdog | 25 July 2010 - 8:49pm

it could be today

yes. Same as you could win the pools today.

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SpaceBoy | 25 July 2010 - 8:59pm

Eggstroardinary

Have you checked the rest yet?

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Slotbadger | 25 July 2010 - 4:43pm

You used to be able to buy...

...boxes of double yoke eggs, so it's definitely occurred before. Pass an egg in front of a strong light source and you can check...Perhaps you have the product of a bored egg-checker?

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nicktf | 25 July 2010 - 5:19pm

Waitrose

as mentioned earlier still do them - we've got a carton on the go now.

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badartdog | 25 July 2010 - 8:40pm

Is this eccentric

or a Hens Egg Trick?

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Badlands | 25 July 2010 - 5:54pm

worst yolk ever

...

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Glenbervie | 25 July 2010 - 9:30pm

Second worst

1
nigelthebald | 25 July 2010 - 10:01pm

Forget the singles...I'm waiting for the double albumen

...preferably on shellac.

(needs work)

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nicktf | 26 July 2010 - 12:08am

I don't who it was but someone

described an egg as a perfectly packaged chicken period, enjoy your double yokers!

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Dave Amitri | 25 July 2010 - 8:56pm

'Cept it's not!

This joke has always bugged me, and not just because I enjoy eating eggs! It's a clever line at first sight, I suppose, but it is applying a mammalian concept to an avian species. Chickens lay eggs whether they are fertilised or not, female humans only expel unfertilised eggs, accompanied by the unrequired uterine lining. It's not really the same thing!

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Merv | 26 July 2010 - 12:20am

4th egg cracked this morning

Another double yolker.

Thanks for the stats links; I haven't got the maths to follow it all but I think I get the gist.

Regarding the "randomness" of the eggs, the box was branded "Very Large Free Range", so I think there's two factors increasing the likelihood - if they're chosen by weight, double yolkers are heavier; and maybe free range hens tend to lay more double yolks?

Anyway, watch this space. I might have an omlette tonight...

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keefus | 26 July 2010 - 8:28am

5th egg cracked

... another double.

I'm unfeasibly eggcited now.

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keefus | 27 July 2010 - 6:02pm

Full House!

All six now. Is there someone I should notify?

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keefus | 30 July 2010 - 8:37am
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