If is true that SQRT(Variance) = SD then one cannot have n-1 in the denominator and the other not but should be same?  SD formula:    Variance formula:
I was asked today in class why you divide the sum of square error by $n-1$ instead of with $n$, when calculating the standard deviation.  I said I am not going to answer it in class (since I didn't wanna go into unbiased estimators), but later I wondered - is there an intuitive explanation for this?!
Machine Learning - Where is the difference between one-class, binary-class and multinominal-class classification? Where is the difference between one-class, binary-class and multinominal-class classification?  If I like to classify text in lets say four classes and also want the system to be able to tell me that none of these classes matches the unknown/untrained test-data.   Couldn't I just use all the methods that I mentioned above to reach my goal? e.g. I could describe C1, C2, C3 and C4 as four different trainings-sets for binary-classification and use the trained models to label an unknow data-set ...  Just by saying, Training-Set for C1 contains class 1 (all good samples for C1) and class 0 (mix of all C2, C3 and C4 as bad samples for C1).  Is unlabeled-data C1 -> 1 or 0  Is unlabeled-data C2 -> 1 or 0 ... and so on ...  For multinominal classification I could just define a training-set containing all good sample data for C1, C2, C3 and C4 in one training-set and then use the one resulting model for classification ...  But where is the difference between this two methods? (except of that I have to use different algorithms)  And how would I define a training-set for the described problem of categorizing data in those four classes using one-class classfication (is that even possible)?  Excuse me if I'm completely wrong in my thinking. Would appreciate an answer that makes the methodology a little bit clearer to me =) My problem is that I have only one reference. So I would like the head of this part to appear "Reference" instead of commonly "References". More generally, could we change the title "References" to an arbitrary name?  Thank you very much for your help! Understanding quantum entanglement.. help me validate this analogy! I'm struggling to understand the concept of quantum entanglement. I've distilled my understanding into an analogy, and I need your help to validate it. Here it is:  Let's say I receive two envelopes. Both envelopes have this written on them:     Hello! Open this letter...   If the paper inside is red, then other envelope has blue paper.   If the paper inside is blue, then other envelope has red paper.   So, if I open one letter, and the paper inside is red, then I know the other letter contains the blue paper.  Is this equivalent to quantum entanglement? The fact that I will know the color of the other paper when I open (i.e. observe) one envelope?  In theory, if one of the letters "appeared" in a different galaxy, I would still know the color of the paper inside that letter, just by opening my letter, instantly, correct?  Before I open the letter, is the paper inside each envelope in superposition (??), i.e. 50/50, red or blue?  In this analogy, I am assuming that what's written on the letters is true. Is this a correct assumption in quantum physics? I think so yes?  In real quantum physics, would I be able to change the color of one paper, so that I change the color of the other one? No right? Splitting lines by polygons in QGIS I have a river network (lines) and a .shp with states (polygon). I would like to split the lines at the state boundaries i.e. split lines by polygon boundaries. Is there a geoprocessing tool to do this in QGIS? I can't merge the lines by the way, since they are meaningfully split into tributaries/main river. Why does resistivity increase with temperature?  The explanations I have heard so far are that increasing temperature increases vibrations in the lattice structure resulting in the number of collisions increasing.  But the vibrations from thermal energy are random, so around half of the vibrations would result in the atoms moving out of the electrons way and half would result in atoms moving into the way of the electrons.  And since half of the vibrations result in increasing the drift speed and half reduce it, shouldn't the average drift speed remain the same? What is the speed of time? The recent user profile tab name change - from "activities" to "all actions" - was not accompanied by a hyperlink update from ...?tab=activities... to ...?tab=allactions...    Click on the "posts edited" link to see the 404-error. How can I get a copy of a Canadian visa refusal letter? I have applied for a Canadian Temporary Resident Visa (TRV); a previous paper application was refused in 2016. I have lost the copy of my refusal letter and want to submit it when I apply for my new visa. How how can I get the a copy of the 2016 refusal letter from Embassy? Achievements tab isn't working in Area 51 When I try to bring down the Achievements tab in Area 51, it's showing an error dialogue.  Strangely enough, the global inbox dropdown works just fine.  Could be connected to .  Screenshot: What is the function of this circuit? Just wondering if someone can give me an explanation for this capacitor layout - what is it's function? The chip to the right is an ADC.  Circled in blue.    I'm guessing it is well known but i don't know how to search for it. If $Y$ is connected, why is $A\cup Y$ connected in this case? I'm glad the outside overlay scrollbars are finally gone.  But the scrollbar is still a razor thin line that becomes a less razor thin scrollbar, but still thin scrollbar, when the mouse hovers.  How can I get a normal scrollbar? I.e., like in other OSes, like in Firefox, a scrollbar that doesn't change depending on what the mouse points to, and with a nice width.  I already tried  gsettings set com.canonical.desktop.interface scrollbar-mode normal   but no luck. I wrote a question about blocking certain characters in passwords. It became tremendously unpopular, and it seemed like everyone wanted to rain on my parade every day.  Eventually, I "got it," agreed completely with what people were saying to me, and just wanted to withdraw my question so that my reputation didn't slide even further downhill.  Unfortunately, the system is designed such that I can't withdraw my question or redeem myself. Instead, I just get daily reputation attacks. That's a terrible system, and it needs to be changed. I add js file with drupal_add_js in hook_form_alter, it is ok and I see it in list of js loaded in page, but when I submit form and form not validate and return to that form (after validation failed) I don't see js files that added to form, why this happened? and how can add my js file in that form after validation failed ?   some part of my code   function mymodle_form_alter(&amp;$form, $form_state, $form_id) {   if ($form_id == 'news_node_form') {     drupal_add_js(drupal_get_path('module', 'mymodule') . '/js/mymodulejsfile.js'); //, array('scope' =&gt; 'footer','weight' =&gt; 10));   } }   code work correctly in add and edit form, but when form submit and validation failed it show validation error but my js file not loaded in page (I checked html source and check firebug script files), but there is not my js file, I am happy to always not understand how does a transformer works ... One answer give me 10 questions ... (So I suggest you to not answer to this question, if you do not want to have more questions :D)  Here is the circuit :     So I put the "coupling" factor to 1, in order to have no leakage inductance and I set the value of the inductors very high in order to have a very low magnetizing current. So the primary current is approximately equal to the secondary current. My question is the following : How the primary current can reach an equilibrium to be equal to the secondary load current ? I will not accept answer which tells Pout is equal to ... So Pin is equal to Pout... There is something happenning into the transformer ...   Thank you very much and have a nice day ! My processor is 64-bit - does that mean I need the amd64 image? Problem of unbalanced data The rules of Transfiguration: examples of making the inedible edible? Information from the official website of J.K.Rowling is generally considered canon. I remember at least one case when she 'changed her mind' after a particular rule has been  published on her website (what happens with a secret if a Secret Keeper dies). Nevertheless, the information from the website can generally be considered reliable.  A rather obscure piece of information is J.K.Rowling's Wizards' Ordinary Magic and Basic Aptitude Test () published on her website in 2007 and now accessible with a Time Turner.   In particular, we know that only one of the following options is correct (as the software allows you to choose only one option):     Which of the following is CORRECT?            a. Food can be conjured out of thin air.       b. Any object can be Transfigured into food.       c. Foodstuffs can be increased, Transfigured, summoned from a distance and magically cooked.       d. It is impossible to make the inedible, edible.   e. Food-related Charms are some of the simplest forms of magic.      W.O.M.B.A.T. Grade 3, 13 June 2007, question 8   The test appeared on J.K.Rowling's website before the release of Book 7, but we can almost safely assume that there was no time for J.K.Rowling to change her mind after compiling the test (though she must have submitted these questions weeks before they were eventually published).  As Hermione explained in Harry Potter and the Deathly Hallows:      Your mother can’t produce food out of thin air, no one can. Food is the first of the  to . It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some...   It means that the correct option is c, and therefore all other statements are false. So, apparently, you can make food out of something else, though not any object can be transfigured into food. Can you come up with any example of such transformation mentioned in the books? How can I compile different .tex file on click on typeset in TeXShop? (MacTeX 2011) My problem is that I want to have different .tex files for the preamble, the presentation aroundings (my target is a beamer PDF file) and the content of the presentation.  Mostly I'll be working on the content file. I even think about partitioning this content .tex file into several chapter files for a better overview. My problem with this solution is that I have to open the presentation.tex file to typeset the whole beamer PDF for looking at the results of my previous changes on the content files. (My presentation.tex file includes the preamble and the content.)  Is there any way to get TeXShop to always typeset one particular file (my presentation.tex) although I'm currently working on a different one (e.g. the content.tex)?  A shortcut? A command in the .tex file? An option in TeXShop? Anything? How can I invest in country A to take advantage of their great interest rate while I live in country B? If I live in the US, for example, where current interest rates are practically nil, how can I take advantage of the fact that ?