Wikipedia:Reference desk/Archives/Science/2007 June 7

= June 7 =

Umbrella cockatoo info needed
Is it true that the personality of an Umbrella Cockatoo changes completely when it reaches breeding age? Mine is nearly three now and people have told me that she won't be anything like the cuddly toy she is now in a couple of years and it will be like owning a different bird.
 * According to Wikipedia, it is now Vulnerable and might well become Endangered. Or not, depending on us. -- DLL .. T 17:01, 7 June 2007 (UTC)
 * "Unsustainable levels of trapping for the cage-bird trade pose the greatest threat. In 1991, an estimated minimum of 6,600 birds (possibly representing a mere quarter of the actual figure) were taken from the wild. Catch quotas for the species were exceeded by up to 18 times in some localities, indicating that trappers were removing in the order of 17% of the population annually." Skittle 21:03, 7 June 2007 (UTC)


 * Guys, guys. Knock it off with the soapboxing - the OP came here to ask a question about his pet's behaviour, not to have a guilt trip laid on him for owning a cockatoo. --Kurt Shaped Box 23:14, 7 June 2007 (UTC)


 * That certainly happens in other animals (not least, humans) - so it's perfectly possible. SteveBaker 18:32, 7 June 2007 (UTC)


 * AFAIK, it varies from bird to bird. I know that sexually mature parrots tend to become temperamental in the spring, when the hormones kick in. If your bird gets it into her head that she wants to build a nest and you try to stop her, you'll probably get bitten. --Kurt Shaped Box 23:24, 7 June 2007 (UTC)

dark energy
I read on your anti-gravity page that one of the things that could mimic its effects was the repulsive effects of dark energy, So if a small amount of this energy could be harnessed [and i know that at this time we are not even close to properly measureing it so i am talking about in principle here] it could be used [put in the most naive and simple terms] for example to push over a chair or a stone along the ground etc, could it actually make a net force on a physical object? thank you


 * Yes - but only in the way that gravity and magnetism could do similar things. This is a source of a force not an energy source - so there is no limitless source of free power here.  SteveBaker 18:29, 7 June 2007 (UTC)

Yea thats what i meant it is a source of force so it could actually move stuff. thanks

What is the possible effect of AIDS on human evolution?
Since AIDS is one of the biggest causes of death today (and especially of young reproducing humans), has there been any discussion of its effect on future human evolution? One can speculate that the character traits of promiscuity or risk taking would be selected against due to AIDS. Neither the articles on human evolution or AIDS discusses this and I could not find anything with an internet search. Delmlsfan 03:00, 7 June 2007 (UTC)
 * If promiscuity or risk taking were genetic, they might be selected against. But they are certainly not only genetic, and the extent to which they are is pretty conjectural. More likely would be the selection for the delta 32 variant of the CCR5 gene, which conveys some measure of immunity. It's not only humans that would evolve, but also the virus, which if it follows the normal pattern of infectious diseases would become increasingly less fatal. - Nunh-huh 03:09, 7 June 2007 (UTC)
 * A couple of points which I consider important would be that AIDS isn't a major cause of death in the developed world and evolution takes place over thousands of years, not in one generation. Further; concepts of "promiscuity" and what you consider "risk taking" would be very, and I mean very different in the places where AIDS is doing the most harm, I mean to the point where I doubt we could even imagine what it is like... Vespine 05:11, 7 June 2007 (UTC)

The question highlights the problem with our understanding of evolution. While there is overwhelming evidence that evolution occurs, it is nearly impossible to use the theory to predict outcomes. For example, you postulate risk taking and promiscuity as being factors that would be selected against. Yet it is just as plausible to suggest that risk takers and promiscuous people would be selected for as they may reproduce in greater numbers at greater frequency. Leaving aside the genetic correlation to these behaviors, both are just as plausible as selection theories and neither outcome would invalidate the theory. That makes the theory virtually useless in a scientific sense. The theory has been expanded and is excellent at predicting "missing links" in the historical fossil record. But it is absolutely misapplied when trying to us it as a predictive measure of selection. --Tbeatty 07:11, 7 June 2007 (UTC)

I don't see why it would be wrong to predict that IF it continues to be a major cause of death for an evolutionarily significant period of time, the genes that confer resistance would become more common. And that time period to be significant fro evolution drops with very high death rates - if over a third of the population is dying of it, natural selection happens pretty fast (maybe only a century or two). Vultur 15:44, 7 June 2007 (UTC)


 * There are lots of complicating factors here - let's just suppose that 'risk taking' is the primary reason people fall into the kinds of situation where they can catch this disease (I'm not saying it is - only that it's one possible thing that evolution might 'fix'):
 * Risk taking might not be genetic at all...if not then you are relying on an anti-risk-taking gene popping up and then being selected for - which might be astronomically unlikely if there is no existing genetic mechanism that could be coopted.
 * Even if there is a 'risk taking' gene - it might be linked to something important that can't "go away" without resulting in a much worse outcome - in that case, it won't be selected for. This was historically the reason that genetic diseases like 'sickle cell aneamia' didn't get selected out - the 'bad' gene also confers greater immunity to malaria.
 * People who are risk takers might get someone else pregnant (or become pregnant themselves if female) with greater frequency than non-risk takers. Hence the fact that they might get AIDS later in life might not prevent their 'risk taker gene' from being selected.  Undertaking the 'risky' behavior of not bothering to wear a condom has two possible consequences - one increases the chances of passing on your genes to the next generation - the other decreases it.  Which one wins?
 * Are risk takers more likely to die for other reasons (eg because they drive cars at higher speed and therefore have more car accidents or something) with a greater probability than from AIDS? If so, what might evolve may not be an avoidance of bad behavior with needles and gay sex, etc - what might evolve could be fear of driving too fast.  That could actually increase the prevelance of risk-taking in the population despite the AIDS consequences.
 * Medical science is acting to increase the lifespan of AIDS patients and to decrease the probability of a mother passing AIDS on to her offspring. Both dramatically weakening the ability of evolution to have an effect - both because treatment gives the person longer to pass their genes on - and because pre-birth treatment prevents the next generation from dying before they get to pass their copy of the gene on.
 * Perhaps Richard Dawkin's concept of the 'meme' might also be at work. Parents who got AIDS but who have children who didn't might be 'scared straight' and teach their kids all about safe sex and so forth more effectively than happens in the general population.  This effect would look a lot like genetics - but actually be a meme at work.
 * SteveBaker 18:23, 7 June 2007 (UTC)

Powdered alcohol
Anyone know how this stuff works, I mean chemically? For those who are too lazy to follow the link, it's a powder that, when mixed with water, creates an alcoholic drink intended for human consumption. It seems to be ready to drink immediately after adding the water, and yields around 3% ethanol, though it seems that might change depending on how much water is added. tucker/ rekcut 03:32, 7 June 2007 (UTC)
 * Wow, I hope someone answers, cuz I'm curious. Do they bind it with a carbohydrate or something? Anchoress 03:44, 7 June 2007 (UTC)


 * This link has a summary of the method:
 * According to the manufacturer, the powder is produced in the following way. A solution of water and alcohol is mixed with maltodextrin and then spray-dried at relatively low temperatures. Each molecule of alcohol is encapsulated by dextrin during the spray-drying process and the smaller water molecules are allowed to pass through the surface and evaporate.
 * Adding water dissolves the maltodextrin and releases the alcohol back into solution. Clever. TenOfAllTrades(talk) 04:02, 7 June 2007 (UTC)
 * i can already see the kids trying to snort it.. Vespine 05:17, 7 June 2007 (UTC)

3% sounds like near beer. 6 proof? Doesn't cough syrup have more? --Tbeatty 07:16, 7 June 2007 (UTC)
 * Many U.S. cough syrups have phased out alcohol but they once were 5 to 50 proof. Here is a list of some from 2000: . Rmhermen 14:07, 7 June 2007 (UTC)

Neural engineering
How do I go about studying this?Do I have to engineering school? Is this the Bioengineering theme or Biomedical theme? 202.174.139.53 05:24, 7 June 2007 (UTC)


 * See Neural engineering and the external links therein. Rockpock  e  t  05:37, 7 June 2007 (UTC)


 * I had a friend who worked on neural networks and she was an electrical engineer. This link has some of the engineering aspects. For a biological approach I am guessing neurobiology would be the field to pursue. It depends on the emphasis that you want to take, either build them or understand them. David D. (Talk) 05:41, 7 June 2007 (UTC)
 * Just looked at Rockpockets links and there are a lot of good ones in there. David D. (Talk) 05:44, 7 June 2007 (UTC)

Biomedical engineering is what you might want to study. &mdash; Scientizzle 08:13, 9 June 2007 (UTC)

time of flight
a body is undergoing free fall under gravity... its infact constrained to move in a pipe.. initial and final position of the body in a 2d plane perpendicular to ground are fixed. initial position is at a higher vertical distance from ground than the final position. we seek to find the shape of pipe such that the time of motion is the minimum possible. attempt at solving... if the initial and final positions were on the same vertical line then a straight line path would be the best to get minimum time of motion. but if the 2 positions are not in same vertical line straight line need not be the best path though it has minimum length,such a path is no longer vertical but inclined and therefore has only a component of g(accelration due to gravity) acting on it.so there is a trade off between getting a high component of g and reducing the length of the pipe we seek for such a shape of pipe which gets the best of this tradeoff.but a detailed mathematical attempt to obtain shape by imposing the condition of minimum possible time gives a very complex condition.perhaps there is a simpler analytical way to obtain the answer. 59.93.40.134 07:50, 7 June 2007 (UTC)


 * Sounds like the Brachistochrone problem to me. Your desired shape is an upside down cycloid.  If I may ask, who was evil enough to give you this problem?  Someguy1221 08:47, 7 June 2007 (UTC)


 * Yes, the trajectory is, indeed, a cycloid. I concocted and asked User59.93.40.134 this question after a visit to a science center where a practical model of the same was on display. Thanks. -Leela


 * And yes, there is a (reasonably) simple analytic way to solve these problems. See calculus of variations and specifically Euler-Lagrange equations. Algebraist 10:11, 7 June 2007 (UTC)


 * I just found out about the related tautochrone curve. Cool! No matter where on the curve you start, you will reach the goal in the same amount of time. —Bromskloss 11:33, 7 June 2007 (UTC)

Lithium-Iron Phosphate Batteries
Do these batteries hold more energy at a temperature of absolute zero? 71.100.163.109 11:38, 7 June 2007 (UTC)

Wal-Mart interior lighting
Does Wal-Mart use some sort of special lighting other than ordinary flourescent lighting that is used inside most large commercial buildings?--Dougsideas 12:10, 7 June 2007 (UTC)


 * Both of our Wal-Marts primarily use skylights (windows in the roof) during the day and regular fluorescent bulbs when it is dark. --Kainaw (talk) 13:20, 7 June 2007 (UTC)
 * At least some of the stores have sophisicated system which actively balances the natural light with fluorescents: Rmhermen 13:52, 7 June 2007 (UTC)


 * I have noticed that not only does Walmart do this, but most newer warhouse type stores. Costco, and Fry's have skylights.  My local grocery store does not though I don't see why not.  Maybe the grocery was built before putting in skylights became standard for big warehouse type buildings. -Czmtzc 14:28, 7 June 2007 (UTC)

Immunoprecipitation(IP)
Is it possible during IP, that when you 'pull out' protein A, you see protein B binding to it, however, when you 'pull out' protein B, you do not see the binding between protein A and protein B? If yes, can anyone please explain to me why this happens?

Thank you very much.


 * The short answer is yes. The reasons are many, and an exhaustive treatment of all of them would be beyond the scope of this Desk.  A few possibilities include:
 * The anti-B antibody is monoclonal and binds to an epitope that is on the A-binding region of protein B. Only B that isn't bound to A will then be collected by the IP.
 * Binding to the anti-B antibody induces a conformational change in B that causes it to release its bound A.
 * B has a large number of possible binding partners; A has only one (protein B). Pulling down protein A will bring down a large amount of B in such a case; pulling down an equal amount of B will bring down only a little bit of A mixed in with all of B's other binding partners, and it will be difficult to detect A.
 * Your anti-A antibody isn't very good, so you don't see it on the B pulldown. Alternatively, your anti-B antibody isn't perfectly specific, so you're seeing a spurious band on the A pulldown.
 * I'm sure there are many other possibilities. Immunochemistry is often a bit of a black art. TenOfAllTrades(talk) 14:21, 7 June 2007 (UTC)

Atomic orbitals, lineair combinations of
Atomic orbitals are solutions for the (time-independent) Schrödinger equation, applied to a central force, right? And so the state of an electron (ignoring their spin) can be descibed by 3 quantum numbers, defining its wave function. Furthermore (and this is confusing me) every lineair combination of these wavefunctions is a solution too? So there must be quite a lot of states for an electron to sojourn in. Do they háve to be lineair independent? If so, why do textbooks sometimes prefer to 'mix' them (eg in p_x and p_y orbitals, in hybridisation,...)? Do electrons have a preference?

If a question makes no sense, please explain as well why not. Thanks --Gnorkel 14:32, 7 June 2007 (UTC)


 * The questions make a lot of sense. So much so that they cover a significant portion of a grad course in atomic physics. The best thing for you to do would be to take Landau & Lifshitz Quantum Mechanics (nonrelativistic theory) textbook and read the relevant chapters; that would be "angular momentum", "motion in centrally-symmetric field", and "atom". If you would like an even more detailed discussion, I would recommend Theory of Atomic Structure and Spectra by Cowan. Now, in brief, the answer is as follows. For a sigle-electron atom or ion, neglecting relativistic effects, there is a degeneracy on angular momentum and on its projection. That is, disredarding the spin part, for every principal quantum number n there are n2 linearly-independent states of the same energy E = Z2 / 2n2. Such states are called degenerate: indeed, since each of them is an eigenvector of the Hamiltonian with the same eigenvalue E, any linear combination of these states is also an eigenvector with eigenvalue E. There are inifinitely many ways to mix these states, but no matter what you do you will never get more than n2 linearly independent ones. Electrons have no preference. When you add another electron to the system, or external field, or include relativistic effects, the degeneracy will be removed at least partially. Hope this helps. Cheers, Dr_Dima.


 * Dr_Dima is right, this question requires a textbook to answer. I'd only like to add one thing, as you mentioned hybridization. These are linear combination of non-degenerate orbital; the 2p orbitals have slightly higher energy than the 1s orbital has. That they get transformed (in the standard example of a carbon atom) to four degenerate sp3 hybrid orbitals is hence not just a choice of textbooks. If you would just look at a linear combination of the 2s and 2p orbitals these were not eigenstates of the atom's Hamiltonian. It is the influence of the binding partners in a covalent binding that makes the degenerate (in case of methan, e.g.), and they are not eigenstates of the carbon atom's Hamiltonian but of the compund's. In other word: hybridized orbitals are not atomic but molecular orbitals. 85.127.39.5 21:55, 7 June 2007 (UTC)

Vitamin K Fortification in foods
I'm currently working in the nutriton labeling department at a major food company. I've been asked to research Vitamin K fortification levels in foods. I was having trouble finding anything, as to what ammounts are legal in the U.S., or if it even is allowed. I guess my question succinctly is:

Is Vitamin K fortification for food products allowed in the United States, and if so, to what level? Onetrickbunny 14:33, 7 June 2007 (UTC)


 * This question is being discussed on the Humanities desk (for some reason). Please don't double post otherwise it all gets very confusing.--Shantavira|feed me 15:42, 7 June 2007 (UTC)


 * I guess this is more of a legal question then a science question Nil Einne 17:21, 7 June 2007 (UTC)

Harvest
When are you supposed to harvest carrots?71.97.31.174 14:46, 7 June 2007 (UTC)


 * See carrots. --Kainaw (talk) 16:19, 7 June 2007 (UTC)


 * The articles states: "Carrots may be harvested at any time until winter." so I don't know if this is an appropriate answer to the question. And it makes me think of why they should not be harvested in winter. For other plants, when they start to build flowers they uses a lot of resources on this and don't have much left to store into the roots. 84.160.213.25 20:27, 8 June 2007 (UTC)

List of genetic disorders
Is there a List of genetic disorders which has the frequency or occurance in the human population on the net? --Juliet 15:41, 7 June 2007 (UTC)
 * As far as I'm aware, there is no know correlation between any genetic element and internet usage nor is the human population using the internet a genetic cluster. Therefore, it's a bit pointless to analyse the frequency or occurance of genetic disorders of the human population using the internet Nil Einne 17:20, 7 June 2007 (UTC)

Let me re-phrase my question: Is there a List of genetic disorders which has the frequency or occurance in the human population?
 * If you find one, it won't be accurate. The occurance of some genetic disorders, especially in the developed world is fairly well known. But for others and especially many of those predominantly affecting the developing world, their frequencies are simply not well studied (even the disorder is probably not well understood, if it's even known). Also, in general it's pretty pointless coming up with frequencies for the entire human population. It's better to know the frequencies for the various human subpopulations and especially to know what subpopulations have a high frequency of a disorder. Nil Einne 17:42, 7 June 2007 (UTC)

OK, how about in the United States? The genetic disorder article cites some examples. I am looking for more examples. If someone else could please answer my question.
 * The United States is generally not going to be studied as a whole either. At the very least, one would consider white/caucasian, blacks etc seperarely. I'm not sure why you're so desperate for frequency information but I suggest you check out the articles for some of the common disorders. You're likely to find some frequency and occurance information in them Nil Einne 17:58, 7 June 2007 (UTC)
 * Nil, I can think of a lot of uses for frequency information for the American population, or the world. It gives insight into how common something is, it allows you to judge priorities, etc. I have seen such information, although not specifically on the internet, as a list of genetic disorder frequencies. It does seem to be hard to find such a list on Google, but maybe someone with more knowledge could use the right search terms. Skittle 20:37, 7 June 2007 (UTC)

The reason should not be of your concern. The genetic disorder article in Wikipedia does not cite race as a factor in any of the disorders. It appears that you have some reluctance in answering my question. Might I suggest that you don't and allow someone else who can.


 * Here is a list for the UK population, I can't find an American equivalent. This database has all the relevent information for many genetic conditions, and the frequency they are found in specific populations (e.g. The prevelence of cystic fibrosis in the North East of the USA in the 1950s was 1 in 2300 ), but it is not provided as a list. Rockpock  e  t  23:33, 7 June 2007 (UTC)
 * Not a list, but you can often find gene frequency data for specific conditions at the Online Mendelian Inheritance in Man database. - Nunh-huh 01:20, 8 June 2007 (UTC)

Brain Transplant
If you took a brain out of the body of an older person, and put it into a the body of a younger person, could the older person's brain live an extra lifetime? Could it be put into an artificial body and live forever? —Preceding unsigned comment added by 199.197.126.242 (talk • contribs)
 * See Whole-body transplant (aka brain transplant) Nil Einne 17:44, 7 June 2007 (UTC)
 * For your last question you may be interested in Ghost in the Shell (philosophy) ;) Vespine 22:29, 7 June 2007 (UTC)


 * This seems to be a philosophical inquiry rather than scientific, at least at this point in time. Any technology that would allow a whole brain transplant into another human body or into an artificial body has not yet been discovered.  So the question might be 'Is this possible?'  Can you logically imagine taking the Brain out of one person and implanting it into another?  Yes.  What consequences would this have?  We don't know.  There are several competing theories concerning Personal identity (philosophy).  One of them suggests that our identity is based in our memories alone.  And if our memories transfer with our brain this would mean the same person would live on after a successful transplant.  However, contradicting theorists might argue that our identity lies within our soul and that such a transplant would not result in the continuation of life for that person.  If a person's identity is somehow tied to and only to their brain, then the answer to your question would be that if such technology existed it would seem as though one's life could be extended by transplant or artificial means.  You might want to consider the ethics behind what would become of the brain that once existed in the "younger person's" body in your original question.  12.127.48.250 05:40, 9 June 2007 (UTC) mrdeath

Robot with just LEDs capable of followwing lights?
Hi. I was looking at this page on tiny "bug-bots". The main bot on that page is a little guy which is apparantly able to follow a light (so the description says). However, the only components are two LEDs, two motors, a circuit board and a power source. How can it follow light? I thought LEDs just emitted light - can the be affected by light? The LED article doesn't mention this. Thanks! --Mary 18:11, 7 June 2007 (UTC)


 * Ah, wait. There is a single mention in the LED article, oddly under the heading "Multi-touch sensing", which mentions that they can act as photoresistors. I expect that this is the solution. Thanks! Mary 18:38, 7 June 2007 (UTC)
 * Yes, theyre pretty much just a p-i-n junction, they can work in both directions.213.48.15.234 14:24, 8 June 2007 (UTC)

dark energy question about nature
I read on your anti-gravity page that one of the things that could mimic its effects was the repulsive effects of dark energy, So if a small amount of this energy could be harnessed [and i know that at this time we are not even close to properly measureing it so i am talking about in principle here] it could be used [put in the most naive and simple terms] for example to push over a chair or a stone along the ground etc, could it actually make a net force on a physical object? Also would dark energy also exist in our atmosphere? thank you —The preceding unsigned comment was added by Special:Contributions/ (talk)


 * We don't know what dark energy is. The leading theory is that it is a property of space itself, in which case it would be essentially impossible to manipulate or concentrate.  The alternative would say it is associated with some tangible (but yet undiscovered) type of exotic matter (e.g. quintessence), in which case it could (at least hypothetically) be concentrated and manipulated.  From the Newtonian prespective, yes it would appear as a force.  (In general relativity, it may or may not act like a force, but then gravity isn't really a force in general relativity either.)  However, the amount of force is neglible except on intergalactic scales.  Dragons flight 18:30, 7 June 2007 (UTC)

Lat and Long conversion
I have lat/long coordinates in deg/min/sec. I need to convert them to degrees and minutes with the seconds behind a decimal point on the minutes. Can I just divide the number of seconds by 60 to get the numbers to the right of the decimal? Dismas |(talk) 21:26, 7 June 2007 (UTC)


 * Yes. &mdash; Sebastian 21:31, 7 June 2007 (UTC)


 * However, coordinates are usually either in degrees-minutes-seconds (DMS) or in decimal degrees. You are using a mixed system of some sort. To convert from DMS to decimal degrees, you will need to divide the minutes by 60 and divide the seconds by 3600 and add them. -Arch dude 02:09, 8 June 2007 (UTC)


 * I'm using what my GPS is giving me. It's degrees/minutes.fraction.  Dismas |(talk) 14:03, 8 June 2007 (UTC)


 * Yes, there's a welter of different lat/long degree formats. Degrees + decimal degrees, degrees + minutes + decimal minutes, and degrees + minutes + seconds + decimal seconds are all popular, and with all sorts of variation on the punctuation, too. (See e.g. Latitude.)—Steve Summit (talk) 02:59, 10 June 2007 (UTC)

Equidae
Do the Equidae contain any other genera than Equus? &mdash; Sebastian 21:30, 7 June 2007 (UTC)


 * Equus is the only still living genus. There are many other genera in Equidae known only from fossils.  Dragons flight 22:22, 7 June 2007 (UTC)


 * (ec) The article odd-toed ungulates claims that Equidae has only one surviving genus. There have been other genera that are now extinct. --mglg(talk) 22:29, 7 June 2007 (UTC)


 * Thank you both. How many other genera are there? (That's something we could add to the article to explain the incomplete classification list.) &mdash; Sebastian 00:04, 8 June 2007 (UTC)


 * Here is a 2005 Science article that contains a nice figure of the Equidae family tree and states that the family has held some three dozen extinct genera and a few hundred extinct species. It refers to to B. J. MacFadden, Fossil Horses: Systematics, Paleobiology, and Evolution of the Family Equidae (Cambridge Univ. Press, New York, 1992) for this information. Note that taxonomy is always in a flux, as research continues an debates flare; here for example is a 2006 article that adds another genus. --mglg(talk) 00:42, 8 June 2007 (UTC)

psychology
if a cognitive psychologis is trying to find out whether more extensive education would protect individual from dimentia and related disorders. is the independent/subject variable the individuals or the dementia.Ĵéņņì


 * Neither as far as I can tell, the dependent variable would be the occurence of dementia, and the independent variable would be level of education. see Dependent and independent variables. -- Diletante 22:25, 7 June 2007 (UTC)


 * (edit conflict) I would think that the independent variable is the amount of education, and the dependent variable is the frequency with which dementia is diagnosed. This is more of a correlational or observational piece of research than an experiment, unless someone plans to randomly assign people to get varying numbers of years of education, then waith 30 to 70 years to see the effect the manipulation had on dementia incidence. Most researchers other than Lewis Terman lack that much patience, and he's been dead over 50 years. He started an observational study on a group of gifted students in the 1920's which is to continue as long as they live. Edison 22:26, 7 June 2007 (UTC)


 * The terms "dependent variable" and "independent variable" are used for controlled experiments. It is unlikely that a cognitive psychologist in today's "publish or perish" environment could undertake a 50-year-long controlled experiment: instead, the psychologist would do a retrospective statistical study. In this case, neither variable is independent (i.e., under control of the experimenter) and the result could not, strictly speaking, prove a causal link, only a correlation. -70.177.166.200 02:03, 8 June 2007 (UTC)


 * Questions on the meaning of these terms in pszchology have been asked, IIRC, at least twice recently on the Reference Desk. It seemsto be a standard homework question. Please up look the old discussions in the archive. Simon A. 07:10, 8 June 2007 (UTC)