In chapter 94, Harry knows about Important Things that we haven't seen him learn.

Harry to Dumbledore:

If the enemy can notice you running off to consult the Weasley twins during class after Hermione was arrested, and find out about their magic map and steal it, then they can wonder why I was guarding Hermione Granger's body.

From Harry's internal monolog:

Obvious problem 1, the Dark Lord is supposed to have made his horcrux in 1943 by killing whatshername and framing Mr. Hagrid.

Harry has never been told about Horcruxes, nor the Marauder's Map. How does he know about them?

Has Harry been busy offscreen?

Or, is this exposition cut from Chapter 86?

(Also, isn't Horcrux capitalized in canon?)

EDIT: Chapter 94 was edited to remove Horcrux references.

You're confusing "evil" with "unsympathetic".

I don't think he is.

Perhaps "evil" here just means a object-level match against some entries in Nega-Frankena's list of of disvalues, including: death, apathy and stasis, sickness and enervation, pain and frustration of all or certain kinds, unhappiness, blight, malcontent, etc; untruth; delusion and lies of various kinds, incomprehension, folly; ugliness, discord, monstrosity in objects contemplated; numbing experience; morally bad dispositions or flaws; mutual contempt, hatred, enmity, defection; unjust distribution of goods and evils; mania and obsession in one's own life; helplessness and experience of impotence; pointless abnegation; enslavement; strife, terror; tedium and repetition; and bad reputation, disgrace, shame, etc.

Fermi's seem essential for business to me. Others agree; they're taught in standard MBA programs. For example:

• Can our business (or our non-profit) afford to hire an extra person right now? E.g., if they require the same training time before usefulness that others required, will they bring in more revenue in time to make up for the loss of runway?

• If it turns out that product X is a success, how much money might it make -- is it enough to justify investigating the market?

• Is it cheaper (given the cost of time) to use disposable dishes or to wash the dishes?

• Is it better to process payments via paypal or checks, given the fees involved in paypal vs. the delays, hassles, and associated risks of non-payment involved in checks?

And on and on. I use them several times a day for CFAR and they seem essential there.

They're useful also for one's own practical life: commute time vs. rent tradeoffs; visualizing "do I want to have a kid? how would the time and dollar cost actually impact me?", realizing that macademia nuts are actually a cheap food and not an expensive food (once I think "per calorie" and not "per apparent size of the container"), and so on and so on.

This would avoid camaraderie, team spirit and reputation management being organisational factors.

Er, why would you want to do this? Do you have a specific management domain in mind, where these things actually don't matter?

If not, perhaps you can just watch what happens in carefully-selected, massively-multiplayer games? Eve, maybe?

I've run meetups on this topic twice now. Every time I do, it's difficult to convince people it's a useful skill. More words about when estimation is useful would be nice. 

In most exercises that you can find on Fermi calculations, you can also actually find the right answer, written down somewhere online. And, well, being able to quickly find information is probably a more useful skill to practice than estimation; because it works for non-quantified information too. I understand why this is; you want to be able to show that these estimates aren't very far off, and for that you need to be able to find the actual numbers somehow. But that means that your examples don't actually motivate the effort of practicing, they only demonstrate how.

I suspect the following kinds of situations are fruitful for estimation:

• Deciding in unfamiliar situations, because you don't know how things will turn out for you. If you're in a really novel situation, you can't even find out how the same decision has worked for other people before, and so you have to guess at expected value using the best information that you can find.
• Value of information calculations, like here and here, where you cannot possibly know the expected value of things, because you're trying to decide if you should pay for information about their value.
• Deciding when you're not online, because this makes accessing information more expensive than computation.
• Decisions where you have unusual information for a particular situation -- the internet might have excellent base-rate information about your general situation, but it's unlikely to give you the precise odds so that you can incorporate the extra information that you have in this specific situation.
• Looking smart. It's nice to look smart sometimes.

Others? Does anyone have examples of when Fermi calculations helped them make a decision?

If there will be effort put into actually building something, we might want to look into what other purposes it could serve, such as virtual (persistent?) meetups, remote CFAR sessions, etc. with features such as a "talking totem" that can be passed to enable audio from that person.

(Split from a previous comment for concept independence)

By the time I'll be teaching I'll have finished my Maths and Further Maths A-Levels and I would also have studied a fair amount of higher material.

Those are all good suggestions. One thing troubling me is that I want to help kids develop their problem-solving skills. Ideally I'd introduce some new puzzle and give them a few hints and they would work it out, but from my limited experience with my 9 year old cousin (who is fairly bright), it either ends up with me giving too little help (so he gets stuck and gives up in frustration) or too much help (so he's just following along with what I'm saying rather than discovering something for himself). How can I best strike the balance?

I know it's old, now, but can you seed the latter again? The swarm's missing about 9% right now.

Well put! Have some internet status points!

I've made a number of updates over the past weeks, so I thought I should write a brief new comment summarizing the material that is now available for download. There are two separate torrent files, both of which contain the entirety of my electronic library, comprising about 4,100 items mostly in pdf format.

One torrent contains all the files uncompressed. You can see the contents of the library and select specific files for downloading. Magnet URI:

magnet:?xt=urn:btih:BEDDF7A5647B634C179EA68EBBBAAA80967D9D1D&dn=LessWrong&tr=udp%3a%2f%2ftracker.openbittorrent.com%3a80%2fannounce&tr=udp%3a%2f%2ftracker.publicbt.com%3a80%2fannounce

The other torrent contains a single, compressed file, which is about 20% smaller in size. Choose this one if you want to download the entire library. Magnet URI:

magnet:?xt=urn:btih:1D845DB543FFF3DE83B66FAA595F1A3D9F42ED42&dn=Library.zip&tr=udp%3a//tracker.openbittorrent.com%3a80/announce