Wikipedia:Reference desk/Archives/Science/2011 February 25

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February 25[edit]

Shine-Dalgarno sequence[edit]

How is this pronounced? "Shine" as in the word "shine"? Nadando (talk) 00:02, 25 February 2011 (UTC)

The Wikipedia article on John Shine, for whom the sequence is part named, gives no pronunciation guide, which weakly suggests that it is indeed not out of the ordinary for someone of apparently Anglo-Saxon heritage, as can be seen here. (talk) 01:18, 25 February 2011 (UTC)

Greenhouse effect from producing methane[edit]

If you plant trees, you are extracting CO2 from the atmosphere. What would happen, in terms of greenhouse effect, if you produce methane with this biomass from the trees and release it into the atmosphere? I know that burning the methane would bring you back to the CO2, but this is a hypothetical question. Quest09 (talk) 01:51, 25 February 2011 (UTC)

Methane is also a green house gas. And it is pound for pound much stronger than CO2. Dauto (talk) 03:31, 25 February 2011 (UTC)
(ec)Methane has a much stronger greenhouse effect than co2, but it doesn't last long. It reacts with oxygen and essentially burns. See Global warming potential. Ariel. (talk) 03:34, 25 February 2011 (UTC)
Yes, I know, but since you are also removing a green house gas from the atmosphere, even if it's to produce other, the question remains, is the balance positive, or negative - in terms of greenhouse effect? Quest09 (talk) 12:17, 25 February 2011 (UTC)
Because methane decays faster than carbon dioxide, you need to specify a timescale. Global warming potential is the time-dependent factor used to compare different gases. It is calculated by mass. One mole of CO2, molar mass 44g, would presumably be converted to one mole of CH4 (methane), molar mass 16g. So one mole of methane has 0.36 the mass and over 20 years around 72*0.36=26 times the global warming potential. Over 500 years one mole of methane has around 7.6*0.36=2.76. So even over 500 years, converting the CO2 to methane would be worse. Unless my sums are wrong, which is fairly likely! --Colapeninsula (talk) 15:31, 25 February 2011 (UTC)
It doesn't matter, because methane decays to carbon dioxide. You'll have a certain period with a much more potent greenhouse gas, and then you'll get back the original greenhouse gas that you were trying to avoid. Releasing methane is always worse than releasing an equivilent amount of CO2. Buddy431 (talk) 17:53, 25 February 2011 (UTC)
Methane does not decay to carbon dioxide. It reacts with hydroxyl radicals and forms CO2 and water. (talk) 22:21, 25 February 2011 (UTC)
I'm just using the terminology that user:Colapeninsula. Maybe "react" or "degrade" is a better term in this case, but it's really just semantics. The point is, methane (a greenhouse gas) doesn't last long in the atmosphere, but in being destroyed, it produces another, less potent but longer lived greenhouse gas, carbon dioxide. In fact, that gas is the same gas that would have been produced if the tree had been burned, and is produced in the same quantities, assuming that carbon is not converted into other compounds. Buddy431 (talk) 01:06, 26 February 2011 (UTC)
Methane capture technology is useful for electricity generation. Anoxic decay (better links available?) of plant matter (for example submersed in water) produces methane, as do methane clathrates. ~AH1(TCU) 19:55, 26 February 2011 (UTC)

Rest mass[edit]

Okay, so suppose two electrons initially at rest begin to repel each other. They will evidently speed up, but the total momentum will remain zero. The total mass will remain zero as well, so that energy is conserved. That means that each electron's mass will remain constant. But . So does that mean the electron's rest mass decreases? (talk) 06:29, 25 February 2011 (UTC)

Why would the total mass of the system be zero? The electron has a non-zero rest mass, see Electron#Fundamental_properties. --Jayron32 06:37, 25 February 2011 (UTC)
Erm, I meant remains constant, sorry! (talk) 07:15, 25 February 2011 (UTC)
Let me answer the question I think you are asking. In the initial state both electrons have potential energy - and the potential energy shows up as mass. Once they repel each other the potential energy gets converted to kinetic energy, which also has mass. So it's not enough to just calculate the mass due to velocity, you also need to include the mass due to potential energy. (Which BTW is almost impossible to do in practice - if someone on the other side of the planet turns on an electromagnet my magnet here suddenly has more potential energy since it could in theory be drawn toward that electromagnet, this is also true of a magnet on the other side of the universe, so in practice it's impossible to include all sources of potential energy.) Ariel. (talk) 07:18, 25 February 2011 (UTC)
Wanting to be able to work out the potential energy without considering all possible long-distance interactions is one good reason that we set the 0 of potential energy at infinity: then if that electromagnet turns on, your magnet merely has the possibility of negative potential energy. (Potential potential energy, I guess.) Its current potential energy doesn't change. Another reason to set that 0 is that it lets us (at least approximately) localize the potential energy within the mass of the object. (It's approximate because the EM/gravitational fields have some of that energy; see my further reply below.) If electrons were defined to have, say, 1 eV of potential energy even at infinite separation, we'd have to count up all the electrons in the universe and subtract that many eV from the measured mass of the electron to find its "true" mass without all of those interactions. --Tardis (talk) 15:53, 25 February 2011 (UTC)

Momentum staying zero doesn't imply that mass is constant. In this case, the momentum vectors are opposite and equal in magnitude, so the mass of the two particles is the same at any point in time (assuming velocities same--which you get from symmetry) but it can decrease or increase. So the mass of the electron decreases, but the rest mass is a constant, like the speed of light in a vacuum. Similarly, you can pass light through water, and its speed decreases, but that doesn't affect the speed of light in a vacuum. ManishEarthTalkStalk 09:37, 25 February 2011 (UTC)

This is one of the many questions where "mass" merely causes confusion. Just say energy, and then throw in if you don't like the units. I don't know what definition of mass you're using when you say it decreases: maybe some sort of "apparent rest mass" derived by removing the kinetic energy and supposing the rest to be rest mass. But in this case that's just the true rest mass plus the electrostatic potential energy, so we can just say that the latter is decreasing and be done with it. If we simplistically assume that the electrons do not radiate, then by simple symmetry the total energy (aka relativistic mass) of the electrons must remain constant because the total over both is conserved and each must always bear half of it. Yet another way of looking at it is that the electrons have their rest masses and the electric field in their vicinity has additional energy (mass/whatever). If you work out the energy of that field (which requires some care around infinities), you'll find that it drops as the charges separate, such that the process may be described as the electrons absorbing some of the energy from the electric field and manifesting it as kinetic energy. --Tardis (talk) 15:53, 25 February 2011 (UTC)
Hmmm, what's sort of odd about that is that when, say, an electron raises or drops a few levels in a nucleus, the light that mediates this is quantized and can be expressed as a particle. But how is the energy stored in the electric field quantized and described? (It has to be photons, but it seems hard to figure out where they are, how many there are, what frequency, and above all what prevents them from escaping) Wnt (talk) 04:01, 27 February 2011 (UTC)

medicine gaity-400[edit]

details about medicine gaity-400 —Preceding unsigned comment added by (talk) 07:33, 25 February 2011 (UTC)

Gaity is a brand name for Gatifloxacin, 400 is the dosage in mg. See the linked article on it for more info - in particular note that it is considered unsafe and has been taken off the market, but still seems to be available in China and India. Unfortunately the article neglects to say what it's used for but it seems to be an antibiotic. Also, I've created a redirect for Gaity. Ariel. (talk) 07:51, 25 February 2011 (UTC)


is Vinyl Flooring pvc or another plastic — Preceding unsigned comment added by Wdk789 (talkcontribs) 09:01, 25 February 2011 (UTC)

Yes, PVC, but with Plasticizers to make it flexible (unlike white PVC water pipes which don't have any). Ariel. (talk) 09:19, 25 February 2011 (UTC)

Surge Protection[edit]

Hi, how much surge protection in power adaptor of consumer electronic devices like a TV or STB should be enough? Thanks? Dearkundan (talk) 10:06, 25 February 2011 (UTC)

It depends entirely on the stability of your AC power. I currently live in a residential area of California, and we "never" have power spikes - the electric current out of our wall plugs is pristine. So I consider surge-protection redundant, (in a sense because I'm trusting my power company). But I've lived elsewhere on other power grids, and also on a home electric generator; in those times, I would pay a lot more attention to the surge protector and/or invest in a uninterruptable power supply for important/expensive electronics. Surge protector explains some of the parameters to worry about - clamping voltage, total surge energy dissipation capacity. Larger total-energy capacity will mean that you can sustain multiple, repeated surges, or one very large surge, but let's be honest - if lightning strikes your transformer or distribution box, no surge-protector has the capacity to absorb and dissipate that level of energy. It's been my experience that a UPS is the best way to keep nasty power lines (particularly, if the power is coming from a generator) from zapping my equipment, but UPSes are much more expensive. Nimur (talk) 16:16, 25 February 2011 (UTC)

how to find hot and cold temp[edit]

calculate hot and cold temprature after 25 orbit .assume a 100kg sherical spacecraft is in an equatorial orbit.How is calculation different for a spacecraft in a 90 degree polar orbit (talk) 11:19, 25 February 2011 (UTC)

Sorry, the Reference Desk will not answer your homework for you. The clues you need will probably be in your textbook or in your notes from class. Matt Deres (talk) 11:38, 25 February 2011 (UTC)

EM wave and magnetic field[edit]

Can a em wave and a magnetic field create a stream of electrons. Like the opposite of a cavity magnetron like when a magnetic field and a stream of electrons creates a em wave —Preceding unsigned comment added by (talk) 11:50, 25 February 2011 (UTC)

Yes. Like most physics, Maxwell's equations are invariant under T-symmetry. (talk) 18:41, 25 February 2011 (UTC)

Is there a example of it happening naturally or artificially. —Preceding unsigned comment added by (talk) 19:19, 25 February 2011 (UTC)

Electron precipitation might count. A radio-signal from Earth (usually man-made, but sometimes caused by lightning), sends an electromagnetic wave or disturbance into the Earth's magnetosphere. The electromagnetic wave interacts with the steady-state magnetic field and the magnetically-confined plasma at high altitudes; and sometimes, electrons "squirt out" and rain down to low altitudes. It's a very complex process, though. Various ground experiments, atmospheric remote-sensing experiments, and satellites have investigated this phenomenon: here's a sensationalized (but scientifically accurate) account of killer electrons from space. Nimur (talk) 19:43, 25 February 2011 (UTC)

Sorry to bother you again but what does 'gyroresonate' mean in this context 'Some of the VLF waves from these transmitters will leak through the ionosphere, couple into the magnetosphere, and gyroresonate with trapped particles leading to particle precipitation.' — Preceding unsigned comment added by Lufc88 (talkcontribs) 19:58, 25 February 2011 (UTC)

The term "gyro-resonance" is a short-name for electron cyclotron resonance. This gyro-resonance describes the most simple type of electron motion in a magnetic field. The electron cyclotron resonance article describes it in more detail, but very briefly, it means that an electron (or other charged particle) flies in a circular or helix-shaped orbit around the magnetic field-lines. This is due to the Lorentz force, which describes how a charge moving through a magnetic field experiences a force perpendicular to the direction it is traveling - causing a circular-shaped orbit. In Earth's magnetic field, the magnetic field lines are non-uniform, because of the shape of the Earth's magnetic field. Also, the exact value of the magnetic field strength varies at any specific location, due to complicated interactions with radio waves and plasma. As a result, the path that each electron "flies" can be very strangely-shaped. Due to gyro-resonance, electrons spiral around in "circles", and due to field-parallel motion, these "circles" get stretched into "coil-shapes" (a helix). The size of those spirals ranges from a few meters, to more than a kilometer in radius. As the electrons also move parallel to the field-lines they follow a curved path along the Earth's dipole field, so they also bounce back and forth between the north- and south- polar regions of the earth, tracing out thousands of kilometers of motion. If, during any of these motions, the electron ever reaches a "very low" altitude (say, about a hundred miles above the ground), it's very likely it will "knock" into a gas molecule in our atmosphere. This causes the electron to lose energy, and so it can no longer sustain its magnetic-field interaction, and falls to earth (usually "zapping" a gas molecule in the ionosphere and creating new electromagnetic waves).
I should say - when a radio wave is in gyro-resonance, it means that the radio wave is at the same frequency as the electron's circular orbit frequency. This is a bit like a matched impedance - it means that energy can flow from the radio signal into the electron. Nimur (talk) 21:18, 25 February 2011 (UTC)

Thanks — Preceding unsigned comment added by Lufc88 (talkcontribs) 21:28, 25 February 2011 (UTC)

Malaria - could I be infected with malaria and not know about it?[edit]

I wanted to donate blood today, but I was sent away to take a test for malaria instead. I lived in India 15 years ago and according to Polish law I have to be tested for malaria before I am allowed to donate blood. Does this regulation make any sense? I assume I would have found out a long time ago if I had been infected with malria so this test will be just a waste of my time and the taxpayers money. Mieciu K (talk) 15:46, 25 February 2011 (UTC)

Makes some sense AFAIK Plasmodium vivax is capable of surfacing after quite long dormancy periods (many years) although the one I had Plasmodium falciparum is not. --BozMo talk 15:56, 25 February 2011 (UTC)
(e/c)P. vivax and such don't go away until you get cured. Chances are you would have noticed, but it's theoretically possible. It's worth noting that blood donation controls are a little on the paranoid side, mostly because people have this irrational fear of diseases from blood transfusion that leads them to refuse transfusions that might save their lives. SDY (talk) 15:57, 25 February 2011 (UTC)
Given that there have been notable instances of diseases transferred by contaminated blood products, a little healthy paranoia is probably warranted: Contaminated haemophilia blood products, HIV-tainted blood scandal (Japan), [1]. Recently, there's been some scare about people getting donated blood from people who later developed vCJD. It's unclear how well (if at all) CJD is transmitted through blood, but most blood officials are erring on the side of caution (rightly so, in my opinion). The BBC article I linked to above details the case of what's believed to be the first vCJD transmittion through contaminated blood (though symptoms never showed themselves, and the man died of other causes). Buddy431 (talk) 17:50, 25 February 2011 (UTC)
Here in Spain, you can donate your blood no matter what, but you also get asked lot of question, including in which countries you have been. It gets tested only after the donation. I don't know if this is a better system, but it seems evident that you need to test for possible hidden risks. (talk) 16:29, 25 February 2011 (UTC)
Well, the blood can only be tested after the donation, can't it? Here in the US, one interesting feature of blood donation is that there's a big box that you can check on one of the forms telling them: "Destroy my blood after I leave". I think the reason for the box is in case a donor hears all the questions ("Have you had homosexual sex in the last X years"; "have you ever traded drugs or money for sex"), and answers "Um, no, certainly not" out of embarrassment or shame, then the donor may be insecure enough to actually go through with the entire donation so the questioner won't detect the donor has done these things. And then make sure not to endanger others. (Yes, some of these questions are becoming a bit controversial.) Comet Tuttle (talk) 18:31, 25 February 2011 (UTC)
No. There's a difference of about 400 ml between a donation and a simply extraction. For testing your blood, they only need a little. Homosexuals are still considered a risk group, although the most logical question would be: how many partners/with or without condom/anal or not. But, I suppose that's too personal for most donors. Anyway, I doubt that blood contaminated with HIV gets through the system. (talk) 22:14, 25 February 2011 (UTC)
Well, to give an idea of the absurdity of it, the U.S. and many other countries ban blood donations from any man who has had sex with a man since 1977. Several countries making reforms to this policy have reduced this period to one year.[2] Such policies, no matter what they turn out to be, are probably a combination of public relations and defensiveness against liability suits, representing largely the relative political power and social status of each group affected, with only a small element of medicine involved. Wnt (talk) 01:55, 26 February 2011 (UTC)
You should read this explanation of why exactly the UK excludes men who have sex with men as donors, which the Terrence Higgins Trust currently supports. It is based on the body of evidence, although the current review of recent evidence may lead to changes. Blood contaminated with HIV can indeed get through the system, even though every donation is tested for it, especially if the donor is only recently infected: this is why they go to such lengths to discourage donations from people who might be infected, and include information on how to get tested for HIV if you think you might be infected (rather than treating blood donation as a test). (talk) 13:59, 26 February 2011 (UTC)

The word I have been looking for is Asymptomatic carrier "a person or other organism that has contracted an infectious disease, but who displays no symptoms." Are there any human asymptic carriers of malaria? I have already donated 4 or 5 times in Poland always stating in the questionare that I lived in India for over 6 months but until today nobody took notice of this. Mieciu K (talk) 16:50, 25 February 2011 (UTC)

Vivax malaria isn't nearly as dangerous as falciparum, which causes almost all of the malaria-related deaths, and the symptoms of malaria aren't very specific and could theoretically be mistaken for some other illness if mild enough or if misdiagnosed. Here in the states, a blood donor is considered "at risk" for one year after travel to a malaria risk area and is not allowed to donate, with the exception of Korea (longer deferral due to some unusual strains in that area). No testing is ever done. Honestly, malaria, especially non-falciparum, is not a big deal for a well-equipped medical system (compared to, say, other transfusion-transmitted infections), as there are several fairly cheap and reliable cures available, so even if someone "got through" it's easy to fix. The tragedy of malaria is that "cheap" cures, like a $4 course of an ACT or the even cheaper synthetic quinine derivatives, are too expensive for many of the people affected. Presumably they use the testing instead of forcing donors to wait after traveling, which means you get more donations if your donor pool is mostly people who travel (most notable in the US with the Armed Forces Blood Program). SDY (talk) 20:47, 25 February 2011 (UTC)
Note that is you carry one sickle-cell anemia gene, you may have a partial resistance to malaria, but still be a carrier. This gene is more common among blacks. See Sickle-cell disease#Genetics. StuRat (talk) 22:50, 25 February 2011 (UTC)
As detailed in the Plasmodium article, the hypnozoite stage can lie dormant for many years, but is less common in the falciparum form. Wnt (talk) 03:20, 26 February 2011 (UTC)
In 2004 after about 25 years of blood donation in the UK my blood was declined in Spain because I had lived in the UK during the 90s. I was told that I was at risk of carrying bovine spongiform encephalitis prions (the incorrectly named mad-cow disease) At that time Spain was free of that disease. Richard Avery (talk) 08:21, 26 February 2011 (UTC)

water survival[edit]

Assuming that you did not drown, die of thirst, or be eaten by sharks, how long could a person survive in the ocean if the water is 85 degF? Googlemeister (talk) 16:03, 25 February 2011 (UTC)

There may be some links leading to a specific answer to that at Hypothermia#Water. WikiDao 17:56, 25 February 2011 (UTC)
Not at 85F. If you had water to drink and a buoyancy jacket I would imagine starvation would be the next barrier at that temp. --BozMo talk 17:59, 25 February 2011 (UTC)
According to this chart, that is correct. :) WikiDao 18:12, 25 February 2011 (UTC)
A question related to this was discussed last year. To recap, the longest authenticated record of survival in open water is 5 1/2 days, by a shipwrecked Japanese sailor in World War II. Looie496 (talk) 18:37, 25 February 2011 (UTC)
I would think that overheating could be a problem during the day. If the water is 85°F, how hot is the air ? And add to that sunlight in the day. If overheating wasn't an issue, then sunburn might be. This could eventually lead to ruptures in the skin and then infections. StuRat (talk) 22:41, 25 February 2011 (UTC)
Sunburn would be a problem. Overheating would impossible in those conditions.
I think overheating might be possible. Specifically, if the air temperature is well over body temperature, humidity is 100%, and if the Sun is shining brightly, and if the people have short, dark hair and maybe dark skin, then their brains might overheat. They could continually dunk their heads underwater to cool off, but might not do so if they fall asleep during the day or if their flotation device prevents this. Splashing water on themselves might help, but as their energy gets lower, from lack of food, they may stop doing this. StuRat (talk) 21:07, 27 February 2011 (UTC)
Excessive jellyfish and any exposure to marine pollution (depending on location) may be another concern. ~AH1(TCU) 19:06, 26 February 2011 (UTC)

Round and round the cell pellet goes.... how fast do you spin yours?[edit]

Is 1000 g not a rather large force to subject cells to in culture? Will it cause many cells to burst? Sigma Aldrich (at a course I went to) and Invitrogen (here) suggest 100-200 g, yet the place where I work routinely uses 1000 g. I'm planning on investigating the effects by counting cells before and after centrifugation, to see if many have "disappeared". In fact, I think I'll go and do it right now. --Seans Potato Business 19:07, 25 February 2011 (UTC)

Didn't seem to make a difference. *shrug* ----Seans Potato Business 19:59, 25 February 2011 (UTC)
Because the cells are inside water (fluid) they don't really experience the full effects of that force. Cells and water have nearly the same density, so while they feel a downward force, so does the water, which counteracts it. The only thing left is the small difference in density from water. Ariel. (talk) 20:02, 25 February 2011 (UTC)

Carbon dating[edit]

On Wikipedia today, I came across a claim that carbon dating once showed that two volcanoes from Hawaii that erupted in 1800/1801 erupted millions/billions of years ago (and that presumably, carbon dating is discredited). A quick websearch showed that this claim was repeated on various young-earth creationist websites. This had me wondering - what is the actual scientific explanation for this phenomenon, as I'm not as willing to dismiss carbon dating out of hand? Kansan (talk) 20:34, 25 February 2011 (UTC)

Carbon dating doesn't work for millions or billions of years anyway. But assuming you mean tens of thousands then if the sample is in water it can mess with the results, making the sample appear to be older than it really is. I've read that if your sample is, or ever was, in water, you can not use carbon dating on it. And unfortunately people are not always careful about this. Ariel. (talk) 20:41, 25 February 2011 (UTC)
How do you carbon-date a volcano, anyway? Doesn't that process require once-living material? APL (talk) 21:07, 25 February 2011 (UTC)
I'm pretty sure all you need is some trapped atmospheric CO2. Although I'm not sure you could get that with volcanic rock, since the gases are from the volcano, not the atmosphere. Ariel. (talk) 21:10, 25 February 2011 (UTC)
See uranium-lead dating and my comment below. Nimur (talk) 21:30, 25 February 2011 (UTC)
Carbon dating measures the carbon in things that use to be made out of plants or use to eat plants. It's used to measure the age of fabrics in archeological sites, frozen mammoths, mummies, etcetera. It is not used on things that are made out of rock, like dinosaur fossils or volcanic rocks, because they never breathed in any carbon isotopes.
Maybe they are dating plants that lived and died on the slopes of the volcano? Ariel. (talk) 21:12, 25 February 2011 (UTC)
Except nobody would do that since they already knew when the volcano erupted - and presumably with a much better margin of error than they'd get with even calibrated carbon-dating. Matt Deres (talk) 21:21, 25 February 2011 (UTC)
That may be a perfect reason to do it: To calibrate the tests. Ariel. (talk) 02:33, 27 February 2011 (UTC)
Comment - it's very common to see the term "carbon dating" used incorrectly. Radiocarbon dating measures the ratio of Carbon-14 to Carbon-12, and is "scientifically useful" only over a few thousands of years. Many other types of radioactive dating exist - our top-level article is radiometric dating. In all cases, the idea is to compare ratios of isotopes measured in a sample against those of a control-group. The assumption is always that the relative abundance or ratio of an isotope depends on the condition the object was in - so if it were buried underground, its expected isotope mix is different than if it were exposed to air (for example). Pop-science publications rarely get the subtleties correct - and often just say "carbon dating" when they really mean some other scientific radiometric assay. Nimur (talk) 21:25, 25 February 2011 (UTC)
It's also common for creationists to use a straw man argument, that by finding one discrepancy in a field, they then claim that the entire field of science is thus discredited. For example, one misidentified fossil is used to "prove" that all fossil evidence is bunk. This is, of course, total nonsense, as a complete T-Rex skeleton is definitely not a deformed elephant, or whatever else they claim. Now, I have no idea if anybody mis-dated a volcano by that long, but, if so, it certainly doesn't invalidate the method, just that particular measurement. StuRat (talk) 22:36, 25 February 2011 (UTC)

Thank you very much for the excellent answers. Kansan (talk) 21:42, 25 February 2011 (UTC)

In my view it is a blunder to try to explain what is wrong with a claim without having a clear and specific statement of the claim in the first place. Could you give a pointer to where on Wikipedia you saw the claim you are talking about? Looie496 (talk) 03:37, 26 February 2011 (UTC)
Fortunately, it isn't in mainspace. It was on Talk:Behemoth, but was removed as a rant unrelated to the page. See here: [3] Kansan (talk) 21:41, 26 February 2011 (UTC)

These two deal with the problem.

And these are creationists trying to interpret the 40 year old measurments :

--Stone (talk) 11:30, 26 February 2011 (UTC)

You might want to check Mark Isaak's Index to Creationist Claims, in particular this page that debunks the claims. [4] and [5] are about similar claims, but the first page has links to where I assume the claim mentioned byt the OP comes from.Sjö (talk) 11:42, 1 March 2011 (UTC)

Singular Value Decomposition and prediction[edit]

I'm working on a project where I have an existing dataset where each record contains some textual data and a metric relating to that data. My plan had been to represent this as a matrix where columns were records and rows were 'terms' (n-grams) - so that each element was the frequency of that term in that record.

I'd then hoped that I could augment this with the metric as an additional row, and use the incremental SVD to determine a relationship between the terms and this metric such that, when given an new (i.e. not in the training dataset) incomplete record where I have only the textual data (which I can represent as a vector of frequencies of terms), I could attempt to predict the metric for that record.

However, whilst I'm more-or-less okay with the SVD in principle, I'm unsure of its use for prediction (so my question here is 'how do I do this?'). Looking at various papers, particularly on product/movie recommendation, it looks like I'd have to add the incomplete record to my matrix and then compute the SVD, then... do something. However, this seems like it would be incredibly slow to work. (talk) 22:08, 25 February 2011 (UTC)

All SVD does is decompose the input vector into a different set of coordinates (an output vector), with the intent that most elements in the output vector are close to zero. The SVD algorithm will produce a matrix that transforms between input- and output- coordinate spaces. The output vector is now in a new coordinate-space, whose unit-vectors are usually pretty abstract (they don't necessarily correspond to any observable property of the input-space).
If your input vector represents word frequency, what exactly are you trying to predict? Nimur (talk) 22:19, 25 February 2011 (UTC)
Yes, please tell us what data you have and what you are trying to predict, then we can recommend an extrapolation or interpolation method. StuRat (talk) 22:29, 25 February 2011 (UTC)
I'm trying to predict the metric. The text field explains an idea, and the metric is a retrospective numerical assessment of the quality of that idea. So, given a dataset of ideas and their qualities, can I discover some relationship between some aspect of the text and the potential quality of the idea it represents. Sorry for the vagueness, I'm not sure precisely what I can disclose. We've tried a few methods so far, and the SVD was recommended to us as an avenue to pursue. (talk) 23:01, 25 February 2011 (UTC)
Singular Value Decomposition can be used to reduce the number of dimensions of the problem se [6] so other methods can be used on the reduced problem but I do not know how it can be applied directly. I think a naive implementation would just reduce to a least square algorithm. You need a Supervised learning algorithm such as Support vector machines. I think the software Weka (machine learning) can be useful to you.--Gr8xoz (talk) 14:30, 26 February 2011 (UTC)
If you can't disclose the actual data, could you come up with a similar example, so we can tell what kind of data you are working with ? StuRat (talk) 21:01, 27 February 2011 (UTC)