A few weeks ago, the Proceedings of the National Academy of Science published an article on a study conducted by a group of Cornell researchers at Facebook. They picked about 600,000 users and then, for a week, a subset of them saw fewer “negative” posts (up to 90% were filtered) than they would otherwise see, a subset saw fewer “positive” posts (same), and a control group got a random subset.

After the week, the users in the “negative” group posted fewer, and more negative, posts, and those in the “positive” group posted more, and more positive, posts.

Posts were classified according to an algorithm called LIWC2007.

The study run contrary to a conventional wisdom that people find it depressing to see on Facebook good things happening to their friends.

The paper has caused considerable controversy for being a study with human subjects conducted without explicit consent. Every university, including of course Cornell, requires experiments involving people to be approved by a special committee, and participants must sign informed consent forms. Facebook maintains that the study is consistent with its terms of service. The highly respected privacy organization EPIC has filed a complaint with the FTC. (And they have been concerned with Facebook’s term of service for a long time.)

Here I would like to explore a different angle: almost everybody thinks that observational studies about human behavior can be done without informed consent. This means that if the Cornell scientists had run an analysis on old Facebook data, with no manipulation of the feed generation algorithm, there would not have been such a concern.

At the same time, the number of posts that are fit for the feed of a typical user vastly exceed what can fit in one screen, and so there are algorithms that pick a rather small subset of posts that are evaluated to be of higher relevance, according to some scoring function. Now suppose that, if N posts fit on the screen, the algorithm picks the 2N highest scoring posts, and then randomly picks half of them. This seems rather reasonable because the scoring function is going to be an approximation of relevance anyway.

The United States has roughly 130 million Facebook subscriber. Suppose that the typical user looks, in a week, at 200 posts, which seems reasonable (in our case, those would be a random subset of roughly 400 posts). According to the PNAS study, roughly 50% of the posts are positive and 25% are negative, so of the initial 400, roughly 200 are positive and 100 are negative. Let’s look at the 100,000 users for which the random sampling picked the fewest positive posts: we would be expecting roughly 3 standard deviations below the mean, so about 80 positive posts instead of the expected 100; the 100,000 users with the fewest negative posts would get about 35 instead of the expected 50.

This is much less variance than in the PNAS study, where they would have got, respectively, only 10 positive and only 5 negative, but it may have been enough to pick up a signal.

Apart from the calculations, which I probably got wrong anyway, what we have is that in the PNAS study they picked a subset of people and then they varied the distribution of posts, while in the second case you pick random posts for everybody and then you select the users with the most variance.

If you could arrange distributions so that the distributions of posts seen by each users are the same, would it really be correct to view one study as experimental and one as observational? If the PNAS study had filtered 20% instead of 90% of the positive/negative posts, would it have been ethical? Does it matter what is the intention when designing the randomized algorithm that selects posts? If Facebook were to introduce randomness in the scoring algorithm with the goal of later running observational studies would it be ethical? Would they need to let people opt out? I genuinely don’t know the answer to these questions, but I haven’t seen them discussed elsewhere.

As of today, I am again an employee of the University of California, this time as senior scientist at the Simons Institute, as well as professor of EECS.

As anybody who has spent time there can confirm, the administrative staff of the Simons Institute is exceptionally good and proactive. Not only they take care of the things you ask them, but they take care of the things that you did not know you should have asked them. In fact at Berkeley the quality of the administration tracks pretty well the level at which it is taking place. At the level of departments and of smaller units, everything usually works pretty well, and then things get worse as you go up.

Which brings me to the office of the Chancellor, which runs U.C. Berkeley, and from which I received my official job offer. As you can see, that office cannot even get right, on its own letterhead, the name of the university that it runs:

Also, my address was spelled wrong, and the letter offered me the wrong position. I can’t believe they managed to put on the correct postage stamp. I was then instructed by the EECS department chair to respond by saying “I accept your offer of [correct terms],” which sounded passive-aggressive, but that’s what I did.

When Twitter started to become popular, I remember thinking that the premise of its service, that its distinguishing feature was its limitation, was ridiculous. (Remember never to ask me for investment advice.)

At the time, I thought that it would be really fun to create a parody site where you could only post one bit messages. Clearly, the site would be called bitter, and when you log in the prompt would ask “Are you bitter?” and if you answered yes your post would be a frowny face, while if you answered no your post would be a smiley face. I went as far as checking that this didn’t seem too hard to pull off in Drupal, to make sure no such parody site existed already, and to see if bittr.com or bittr.net were available. (Of course they weren’t!)

Anyways, I was mistaken in thinking that two possible messages, and hence one bit of information, was the end of the road. Indeed, it is possible to have only one possible message, and this is the insight pursued by yo, which, apparently, is not a parody and has received one million dollars in funding.

“I may have the genetic coding that I’m inclined to be an alcoholic, but I have the desire not to do that – and I look at the homosexual issue the same way”

(Rick Perry, Governor of Texas)

So, if I understand Perry’s point, he may have a genetic inclination to be gay but he forces himself “not to do that”?

Judge Rolf Treu has ruled that teachers’ tenure violates California students’ constitutional right to an education.

The California Constitution, which I am now reading for the first time, has an entire section on education (also, an entire section on water), it specifically mandates state funding for the University of California, and it states, right at the beginning, a fundamental right to privacy. (Using the word “privacy,” which does not occur in the US constitution.) Yay California Constitution!

Back to Judge Treu, he has found that the right to an education implies a right to good teachers, and that tenure causes students to have bad teachers. Hence tenure is unconstitutional. Also the bad teachers disproportionally end up in districts with poor and minority students, so tenure is unconstitutional also on equal protection grounds. I am now looking forward to conservative California judges striking down other laws and regulations that affect the educational opportunities of poor and minority students, including prop 209.

I have always known that I don’t completely understand how fiat money works, however I have recently realized that I don’t understand it at all! Maybe some of my readers can clarify my confusion.

So let’s start a national economy from scratch, without international trade: so a group of people move to a deserted island, declare independence, the people have all kind of skills, they bring all kind of materials and machineries with them, the island has all kinds of natural resources and maybe a lottery system gives some people ownership of various plots of lands and mining rights.

Now some would-be entrepreneurs would like to begin hiring people with the right skills, buying or renting various stuff, and start some businesses. It seems that there is no loss of generality if I think of the entrepreneurs as just one person for the sake of what I want to think about. She is going to need to get a loan to start her business(es). Meanwhile, the new island government created a central bank, which issues theory dollars, or thollars, which are the currency of the island. The central bank “creates” thollars and lends them to banks, then the banks keep a fractional reserve and lend to the entrepreneur. Again, for the sake of what I want to say, there is no loss of generality if I identify the central bank and the other banks as just one entity.

So the central bank lends thollars to the entrepreneur, and she uses the money to start the business and being to pay people. At this point money starts circulating in the economy, and people will hire gardeners and babysitters, and lawyers, they will give to charities, they will hire computer science theory tutors for their kids, they will buy and sell houses to each others, and, crucially, will buy whatever goods and services the entrepreneur is selling. Now she is making a profit and she can pay back the loan to the central bank and invest in more … wait, she can never pay back the loan!

That’s because all the thollars in circulation are the ones that the central bank lent her, so there is no way that, as the money circulates, she can ever make more money that she owes!

Ok, so maybe people will also take loans to buy houses and stuff, so that’s more money that circulates, but is it really the case that, overall, it is impossible for everybody to be debt-free? That all the cash that any debt-free person has needs to be compensated by an equivalent amount of debt from other people? This is not how things seem to be in practice.

Now, clearly it is possible for everybody to have positive net worth, because, at the start, people have stuff and that stuff is worth money, but it seems strange that not everybody can be debt-free.

Maybe the problem is deflation? That if the entrepreneur borrowed money to create a business that creates new wealth (because it makes stuff that people find more valuable than the value of the raw material and the value of the work that went into it), but the amount of circulating money stays the same, then there is deflation, and her debt is spiraling out of control in deflation-adjusted terms, even if the interest rate is zero?

It seems that there are only two ways in which you can have everybody be debt-free and have a positive amount of cash: (i) the central bank starts buying stocks of private companies, (ii) the government runs a deficit, and the central bank buys government debt.

Is this correct? Normally, we think of people and companies being debt-free (or having more cash than debt) as ideal, and a government running no deficit as ideal, and usually central banks don’t buy stocks (they only buy bonds, which is formally equivalent to lending), so are these three conditions contradictory?

Where you least expect them:

a common [definiton] for “population” is a geographical cluster of people who mate more within the cluster than outside of it

As you may remember, a few months ago Dieter van Melkebeek, the steering committee chair of the conference on computational complexity, started a discussion on the future of the conference and on its relation to IEEE.

A poll among CCC former participants showed that 97% of respondents favored change, and a majority wanted the conference to be independent of both IEEE and ACM. The steering committee, subsequently, voted unanimously to make the conference independent.

The steering committee is now working out the logistics, and volunteers are needed to help. Already several people have pledged to contribute in various forms, and if you are interested there will be organizational meetings in Vancouver during CCC 2014. (By the way, today is the deadline for early registration.)

I would like to publicly thank Dieter both for the effort that he put on making this change happen and for the transparency of the process. I hope that, if some big change is coming for STOC or FOCS, it will be the result of a similarly open discussion.


The great earthquake of 1906 struck San Francisco on April 18, around 5 in the morning. While the earthquake already caused a lot of damage, it was the subsequent fire that ravaged the city: the earthquake had broken the water pipes, and so it was impossible to fight the fire because the hydrants were not working. Except for the hydrant at Church and 20th, which saved my house and a good part of the mission. The hydrant is painted golden, and once a year, on the anniversary of the earthquake, the fire department repaints it and leaves a token of appreciation. (They actually do it at 5 in the morning.)

By the way, there are two faults that can cause earthquakes in the San Francisco Bay Area. One is (our stretch of) the San Andreas fault, which runs close to the ocean, and which caused the 1906 quake and the 1989 one, and which may not be an imminent risk given the energy released in 1989. The other is the Hayward fault, which runs near Berkeley. The Hayward fault had big earthquakes in 1315, 1470, 1630, 1725, and 1868, that is about every 100-140 years, with the last one being 146 years ago…


25 years ago on April 15, Hu Yaobang died. The day before his funeral, about 100,000 people marched to Tiananmen square, an event that led to the occupation of the square, and which culminated in what in mainland China used to be referred to as the “June 4 events,” and now as the “I don’t know what you are talking about” events.

Also, something happened, according to tradition, 1981 years ago.

I was having dinner at Machne Yuda, that was recommended to me as one of the best places to eat in Jerusalem (although I liked Chakra much better), and I was sitting at the bar, cramped between the French gay couple and the drunk Israeli lady with the huge boyfriend.

At the same time that I got my dessert, the lady with the huge boyfriend stood up to leave and, with remarkable swiftness for a drunk lady, took a piece of my cake asking, while she was already doing so, if she could try it.

Now, between the fact of the ground that a piece of my cake was already on her fork, the size of the boyfriend, the fate of the last people who tried to fight with the Israeli about what is whose and, really, haven’t the Jewish people suffered enough already?, the only logical thing to say was, sure, help yourself.

A little bit later, the bartender presented me with another (different) dessert. “This is on the house,” the bartender said, “I saw what she did, and it wasn’t right.”

The second dessert was better.



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