All Questions
193,830
questions
1240
votes
27
answers
822k
views
Making sense of principal component analysis, eigenvectors & eigenvalues
In today's pattern recognition class my professor talked about PCA, eigenvectors and eigenvalues.
I understood the mathematics of it. If I'm asked to find eigenvalues etc. I'll do it correctly like ...
- 12.7k
745
votes
11
answers
924k
views
How to choose the number of hidden layers and nodes in a feedforward neural network?
Is there a standard and accepted method for selecting the number of layers, and the number of nodes in each layer, in a feed-forward neural network? I'm interested in automated ways of building neural ...
- 52.2k
613
votes
12
answers
439k
views
What is the difference between "likelihood" and "probability"?
The wikipedia page claims that likelihood and probability are distinct concepts.
In non-technical parlance, "likelihood" is usually a synonym for "probability," but in statistical usage there is a ...
- 6,991
540
votes
11
answers
614k
views
What is the difference between test set and validation set?
I found this confusing when I use the neural network toolbox in Matlab.
It divided the raw data set into three parts:
training set
validation set
test set
I notice in many training or learning ...
- 6,919
529
votes
15
answers
220k
views
What is the intuition behind beta distribution?
Disclaimer: I'm not a statistician but a software engineer. Most of my knowledge in statistics comes from self-education, thus I still have many gaps in understanding concepts that may seem trivial ...
- 9,420
526
votes
3
answers
388k
views
Relationship between SVD and PCA. How to use SVD to perform PCA?
Principal component analysis (PCA) is usually explained via an eigen-decomposition of the covariance matrix. However, it can also be performed via singular value decomposition (SVD) of the data matrix ...
- 94.4k
525
votes
23
answers
280k
views
Why square the difference instead of taking the absolute value in standard deviation?
In the definition of standard deviation, why do we have to square the difference from the mean to get the mean (E) and take the square root back at the end? Can't we just simply take the absolute ...
- 5,495
479
votes
20
answers
168k
views
The Two Cultures: statistics vs. machine learning?
Last year, I read a blog post from Brendan O'Connor entitled "Statistics vs. Machine Learning, fight!" that discussed some of the differences between the two fields. Andrew Gelman responded favorably ...
422
votes
5
answers
160k
views
How to understand the drawbacks of K-means
K-means is a widely used method in cluster analysis. In my understanding, this method does NOT require ANY assumptions, i.e., give me a dataset and a pre-specified number of clusters, k, and I just ...
- 6,397
414
votes
14
answers
266k
views
Bayesian and frequentist reasoning in plain English
How would you describe in plain English the characteristics that distinguish Bayesian from Frequentist reasoning?
- 4,249
396
votes
11
answers
171k
views
Explaining to laypeople why bootstrapping works
I recently used bootstrapping to estimate confidence intervals for a project. Someone who doesn't know much about statistics recently asked me to explain why bootstrapping works, i.e., why is it that ...
- 4,929
388
votes
18
answers
155k
views
What happens if the explanatory and response variables are sorted independently before regression?
Suppose we have data set $(X_i,Y_i)$ with $n$ points. We want to perform a linear regression, but first we sort the $X_i$ values and the $Y_i$ values independently of each other, forming data set $(...
- 3,551
382
votes
7
answers
361k
views
When conducting multiple regression, when should you center your predictor variables & when should you standardize them?
In some literature, I have read that a regression with multiple explanatory variables, if in different units, needed to be standardized. (Standardizing consists in subtracting the mean and dividing ...
- 4,231
378
votes
9
answers
742k
views
What is the difference between fixed effect, random effect and mixed effect models?
In simple terms, how would you explain (perhaps with simple examples) the difference between fixed effect, random effect and mixed effect models?
- 5,538
376
votes
26
answers
135k
views
Python as a statistics workbench
Lots of people use a main tool like Excel or another spreadsheet, SPSS, Stata, or R for their statistics needs. They might turn to some specific package for very special needs, but a lot of things can ...
370
votes
80
answers
175k
views
What is your favorite "data analysis" cartoon?
Data analysis cartoons can be useful for many reasons: they help communicate; they show that quantitative people have a sense of humor too; they can instigate good teaching moments; and they can help ...
365
votes
7
answers
1.5m
views
How to normalize data to 0-1 range?
I am lost in normalizing, could anyone guide me please.
I have a minimum and maximum values, say -23.89 and 7.54990767, respectively.
If I get a value of 5.6878 how can I scale this value on a scale ...
- 4,021
358
votes
12
answers
349k
views
Difference between logit and probit models
What is the difference between Logit and Probit model?
I'm more interested here in knowing when to use logistic regression, and when to use Probit.
If there is any literature which defines it using ...
- 5,844
357
votes
16
answers
128k
views
Is normality testing 'essentially useless'?
A former colleague once argued to me as follows:
We usually apply normality tests to the results of processes that,
under the null, generate random variables that are only
asymptotically or ...
338
votes
5
answers
356k
views
What is the trade-off between batch size and number of iterations to train a neural network?
When training a neural network, what difference does it make to set:
batch size to $a$ and number of iterations to $b$
vs. batch size to $c$ and number of iterations to $d$
where $ ab = cd $?
To ...
- 42.7k
331
votes
8
answers
127k
views
Why is Euclidean distance not a good metric in high dimensions?
I read that 'Euclidean distance is not a good distance in high dimensions'. I guess this statement has something to do with the curse of dimensionality, but what exactly? Besides, what is 'high ...
- 3,527
319
votes
13
answers
184k
views
How to understand degrees of freedom?
From Wikipedia, there are three interpretations of the degrees of freedom of a statistic:
In statistics, the number of degrees of freedom is the number of
values in the final calculation of a ...
- 17.9k
305
votes
8
answers
255k
views
What should I do when my neural network doesn't learn?
I'm training a neural network but the training loss doesn't decrease. How can I fix this?
I'm not asking about overfitting or regularization. I'm asking about how to solve the problem where my ...
- 78.1k
296
votes
10
answers
170k
views
What's the difference between a confidence interval and a credible interval?
Joris and Srikant's exchange here got me wondering (again) if my internal explanations for the difference between confidence intervals and credible intervals were the correct ones. How you would ...
- 5,637
291
votes
8
answers
204k
views
Bagging, boosting and stacking in machine learning
What's the similarities and differences between these 3 methods:
Bagging,
Boosting,
Stacking?
Which is the best one? And why?
Can you give me an example for each?
- 3,019
283
votes
16
answers
96k
views
Why does a 95% Confidence Interval (CI) not imply a 95% chance of containing the mean?
It seems that through various related questions here, there is consensus that the "95%" part of what we call a "95% confidence interval" refers to the fact that if we were to exactly replicate our ...
- 12.8k
282
votes
16
answers
527k
views
What is the meaning of p values and t values in statistical tests?
After taking a statistics course and then trying to help fellow students, I noticed one subject that inspires much head-desk banging is interpreting the results of statistical hypothesis tests. It ...
- 4,166
276
votes
152
answers
144k
views
Famous statistical quotations
What is your favorite statistical quote?
This is community wiki, so please one quote per answer.
275
votes
6
answers
432k
views
What does AUC stand for and what is it?
Searched high and low and have not been able to find out what AUC, as in related to prediction, stands for or means.
- 3,129
274
votes
2
answers
213k
views
Interpretation of R's lm() output
The help pages in R assume I know what those numbers mean, but I don't.
I'm trying to really intuitively understand every number here. I will just post the output and comment on what I found out. ...
- 4,191
272
votes
6
answers
541k
views
What is batch size in neural network?
I'm using Python Keras package for neural network. This is the link. Is batch_size equals to number of test samples? From ...
- 3,653
270
votes
6
answers
41k
views
Is $R^2$ useful or dangerous?
I was skimming through some lecture notes by Cosma Shalizi (in particular, section 2.1.1 of the second lecture), and was reminded that you can get very low $R^2$ even when you have a completely linear ...
- 9,190
268
votes
13
answers
207k
views
Is there any reason to prefer the AIC or BIC over the other?
The AIC and BIC are both methods of assessing model fit penalized for the number of estimated parameters. As I understand it, BIC penalizes models more for free parameters than does AIC. Beyond a ...
- 17.2k
265
votes
10
answers
152k
views
How would you explain covariance to someone who understands only the mean?
...assuming that I'm able to augment their knowledge about variance in an intuitive fashion ( Understanding "variance" intuitively ) or by saying: It's the average distance of the data ...
- 13.5k
264
votes
11
answers
180k
views
How would you explain Markov Chain Monte Carlo (MCMC) to a layperson?
Maybe the concept, why it's used, and an example.
- 9,372
256
votes
3
answers
25k
views
How to know that your machine learning problem is hopeless?
Imagine a standard machine-learning scenario:
You are confronted with a large multivariate dataset and you have a
pretty blurry understanding of it. What you need to do is to make
predictions ...
Tim♦
- 113k
253
votes
15
answers
277k
views
What are the differences between Factor Analysis and Principal Component Analysis?
It seems that a number of the statistical packages that I use wrap these two concepts together. However, I'm wondering if there are different assumptions or data 'formalities' that must be true to use ...
- 6,692
248
votes
46
answers
26k
views
What are common statistical sins?
I'm a grad student in psychology, and as I pursue more and more independent studies in statistics, I am increasingly amazed by the inadequacy of my formal training. Both personal and second hand ...
246
votes
7
answers
175k
views
How to choose a predictive model after k-fold cross-validation?
I am wondering how to choose a predictive model after doing K-fold cross-validation.
This may be awkwardly phrased, so let me explain in more detail: whenever I run K-fold cross-validation, I use K ...
- 4,315
233
votes
4
answers
105k
views
ROC vs precision-and-recall curves
I understand the formal differences between them, what I want to know is when it is more relevant to use one vs. the other.
Do they always provide complementary insight about the performance of a ...
- 17.8k
232
votes
38
answers
140k
views
What is the best introductory Bayesian statistics textbook?
Which is the best introductory textbook for Bayesian statistics?
One book per answer, please.
230
votes
8
answers
111k
views
Algorithms for automatic model selection
I would like to implement an algorithm for automatic model selection.
I am thinking of doing stepwise regression but anything will do (it has to be based on linear regressions though).
My problem ...
- 2,450
223
votes
9
answers
103k
views
Why is Newton's method not widely used in machine learning?
This is something that has been bugging me for a while, and I couldn't find any satisfactory answers online, so here goes:
After reviewing a set of lectures on convex optimization, Newton's method ...
- 2,221
223
votes
4
answers
325k
views
When (and why) should you take the log of a distribution (of numbers)?
Say I have some historical data e.g., past stock prices, airline ticket price fluctuations, past financial data of the company...
Now someone (or some formula) comes along and says "let's take/use ...
- 13.5k
219
votes
4
answers
410k
views
How to interpret a QQ plot
I am working with a small dataset (21 observations) and have the following normal QQ plot in R:
Seeing that the plot does not support normality, what could I infer about the underlying distribution? ...
- 18.5k
219
votes
13
answers
178k
views
How should I transform non-negative data including zeros?
If I have highly skewed positive data I often take logs. But what should I do with highly skewed non-negative data that include zeros? I have seen two transformations used:
$\log(x+1)$ which has the ...
- 52.2k
219
votes
13
answers
201k
views
What is the difference between data mining, statistics, machine learning and AI?
What is the difference between data mining, statistics, machine learning and AI?
Would it be accurate to say that they are 4 fields attempting to solve very similar problems but with different ...
217
votes
5
answers
86k
views
Which "mean" to use and when?
So we have arithmetic mean (AM), geometric mean (GM) and harmonic mean (HM). Their mathematical formulation is also well known along with their associated stereotypical examples (e.g., Harmonic mean ...
- 13.5k
207
votes
17
answers
202k
views
Intuitive explanation for dividing by $n-1$ when calculating standard deviation?
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 ...
- 20k
205
votes
8
answers
454k
views
In linear regression, when is it appropriate to use the log of an independent variable instead of the actual values?
Am I looking for a better behaved distribution for the independent variable in question, or to reduce the effect of outliers, or something else?
- 2,201