# FAQ

- When we compute the error (1) of a hold out set, which sigma shall
we use: the same one as in the training set, or we compute a sigma
value for each point in the test set and then average them?
According with Girolami’s book, there is a formula to estimate the
uncertainty (variance) of a single test point once we obtained w*,
but I’m not sure if it’s good to use it to compute the error in the
hold out set.\
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**You fit sigma on the training set** - I’ve found that some authors consider the error function just as the
sum of the differences between actual values and predicted values.
They do not use the sigma term. Which formulation shall we use?\
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**I mentioned this in class. The sigma term comes if you look at the error as the negative log likelihood (which is what we are doing). If you just view least squares as the approach it doesn’t appear.** - How many assignments and exams are there in the course?\
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**Two assignments (one from Neil, one from Trevor), worth 40% (20% each), one exam, worth 60%.** - When is the expected date for the final exam? I may go to my home
country for holidays in December/January, so I just want to make
sure to be back on time.\
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**That isn’t scheduled by us. We don’t have any control over it. It is within the semester period, and you are expected to be in Sheffield across all semester periods. For more details see http://www.sheffield.ac.uk/ssid/exams/timetables.** - The doubt is about the term \(\mathbf{w}\) ; in the linear
regression equation it makes perfect sense about \(w_1\) being
the slope and \(w_0\) being the intercept on the y-axis but as we
move to non-liner i.e quadratic or higher orders what would the
vector elements in \(\mathbf{w}\) stand for i.e. the coefficients
in the equation \(w_0 + w_1 x + w_2 x\^2\) stand for ?\
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I am sorry if this is a stupid question but i needed a little
clarification. Linear combination does tell me the scalars * the
vectors but I am a little confused as to what the scalars stand for
in higher order polynomials.\
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**It’s not a stupid question, because actually those parameters start to lose interpretation as the model becomes more complex. Sometimes they mean something (like in a quadratic,**\(w_2\)**would be the curvature) but in general they don’t necessarily mean anything. It depends very much on the basis set we consider.** - Just a simple question. For question 1 part a) of the first
assignment, we need to compute the error of the fit. Shall we use
the quadratic loss to compute it, or the equation where we need to
fit the variance first?\
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**You should use the error function as it was defined in the class.** - Just a brief question: in part 3 (Bayesian analysis) of the
assignment, how can I plot one standard deviation above and below my
fit? I computed the covariance matrix \(\mathbf{C}_w\), but from
all the numbers, which ones are the ones I need to use for
plotting?\
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**As it explains in the lecture slides from the Gaussian process session (and I went through on the board, and is reviewed in some detail in the Week 5 lab sheet), the variance of the output can be derived as**\[\Phi \mathbf{C}_w \Phi \^\top\]**But that gives the full covariance of the function. For the marginal standard deviations you first need the marginal variances, they can be extracted from the diagonal of the full covariance. Then you need to square root them to get the standard deviations. Error bars are generally shown at two standard deviations.**\ \ - ) I’m having problems to pause a plot and wait until a key is
pressed. I haven’t found a proper command for this. So far, I’ve
made a simulation where a fit is plotted as the number of basis
increases. The title of the plot changes to show to the user the
computed error respectively. Is it fine to leave it like this?\
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**Don’t worry too much about this and with ipython notebook there are other ways of doing the same thing (e.g. displaying several plots in line).** - Once the assignment is finished, where can I upload it?\
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**On MOLE.** - For the Gaussian Process analysis, how can we compute the VC
dimension?\
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**That is not part of the syllabus, GP analysis is rarely justified via VC dimension, which was originally developed for classification. We haven’t taught any statistical learning theory in the regression part of this course.** - For the \(\sigma\) fit that we got using MLE, isn’t it a biased
estimate? Shouldn’t we use an unbiased estimate?\
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**I haven’t covered the concept of biased estimates in this module and don’t intend to (although I did talk about error due to bias and error due to variance in the lecture on generalization, but that’s a different thing), so for this course you should***always*use the maximum likelihood estimate for \(\sigma\^2\) (which is, indeed, biased). - Suppose we’ve a data set. We divide it into a training set and
validation set. We train different models and select the most
appropriate using the validation set. Once we’ve selected a model,
which parameters should we use and report? The ones obtained from
the training set? Or, using the selected model, we “train” again
that particular model but this time with the whole data set?\
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**Good question, it shows you’re thinking. I would be happy with either, but I think the second is best. Although in practice you might do the first because of computational issues.** - For the Bayesian approach discussed in class, you mentioned that the
variance of the output is obtained from
\(\boldsymbol{\Phi}\mathbf{C}_w\boldsymbol{\Phi}\^\top\).
Aren’t we forgetting to add the noise term, \(\sigma\^2
\mathbf{I}\).\
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**Again, this shows you are thinking about it. The posterior (co)variance of \(f(\mathbf{x})\) is given by \(\boldsymbol{\Phi}\mathbf{C}_w\boldsymbol{\Phi}\^\top\), but the posterior covariance of \(y(\mathbf{x}) = f(\mathbf{x}) + \epsilon\) where \(\epsilon \sim \mathcal{N}(0, \sigma\^2)\) is indeed given by \(\boldsymbol{\Phi}\mathbf{C}_w\boldsymbol{\Phi}\^\top + \sigma\^2\mathbf{I}\). I did cover this in class, but may have been too quick.** - During last lab session, why even though we add the variance term to
the GP, the output looks noiseless compared with the Bayesian
output?\
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**If I understand your question correctly, that’s because there is a lot of noise in the uncertainty about \(f\) so adding the noise only makes a small difference.** - Also, on the last lab session, as the GP output moves away from the
data, the response tends to our prior belief with mean zero and
standard deviation L. Is it accurate this observation?\
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**Yes, that’s true, and in this case it shows that the prior wasn’t very sensible, we should have had a prior that doesn’t return to mean zero. As an independent exercise think about a few different ways of doing this.**\ \ - How can we select a good prior distribution to improve the learning
process?\
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**Ah! The question to end all questions! That is a very open question, and it is, indeed, what much of my research is about! It is either that or, given an appropriate prior and likelihood (which together I would call the model) how can I do the Bayesian inference to infer the posterior distributions and the model parameters.**