# All Questions

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### The sum of two symmetric random variables is symmetric

If $X$ and $Y$ are two random variables with probability density functions which are symmetric around their respective means, their sum, $X+Y$, has a probability density function which is symmetric ...
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### What do I do with zero columns in a tensor.prod.model.matrix?

Suppose my two marginal bases are given by (for variable $p$ and $t$ the degree is equal to $1$ and $5$ inner knots are used). ...
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### Converting a set of tweets into vectors for clustering

I have a large set of tweets to which i plan to use cosine similarity to cluster the tweets. I found NLTK's GAAC to be good but how do i convert the tweets into vectors? In ...
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### Efficient evaluation of multidimensional kernel density estimate

I've seen a reasonable amount of literature about how to choose kernels and bandwidths when computing a kernel density estimate, but I am currently interested in how to improve the time it takes to ...
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### How to estimate confidence level for SVM or Random Forest?

I have two classes (say 1 and 0), and want to build a classifier. It is possible to use ...
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### What is the relationship between the GINI score and the log-likelihood ratio

I am studying classification and regression trees, and one of the measures for the split location is the GINI score. Now I am used to determining best split location when the log of the likelihood ...
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### Using a Decision Tree Algorithm such as C4.5 to understand population Partition

I have multivariate data about a certain population with more than 1000 attributes per exemplar. Some of the variables are basic demographics attributes including: gender, age, race, ethnicity, ...
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### Is Big Data the end of Statistics?

Statistics emerged as a discipline whose goal was to allow inferences about inaccessible populations from data obtained from its samples. Does modern access to huge amounts of data depreciate the ...
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### How to get the overall effect for linear mixed model in lme4 in r?

I would like to get the significant value and effect size of the independent variable in overall, rather than the normal output from lme4 in R. It is just like the thing people report when running ...
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### Mean ranks and standard errors of Kruskal-Wallis test

While it's easy to compute the p-value of Krsukal-Wallis non-parametric test using the kruskal.test() function, I don't know how to compute the mean ranks and ...
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### How to get confidence intervals for specific indirect effects in Amos?

Say I have an SEM model with 1 predictor variable (IV), 2 mediators (MV1, MV2) and 1 dependent variable (DV). Amos reports combined indirect effects for an IV on a DV. So the combined indirect effect ...
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### Calculate standard errors: interaction between 2 factors, one of which has 3 levels in a regression model

I have the following model: $y \sim b_0 + b_1x_1 + b_2x_2 + b_3x_1x_2$. $x_1$ is a factor with 2 levels (0 and 1), and $x_2$ is a factor with 3 levels. I know that to calculate standard errors for ...
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### regression when independent variable is the difference betwen two proportions

Here is some sample data ...
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### analysing difference in variability between multiple related time series

I have constant time series (yearly from 1950 to 2010) of the abundance of several species that were captured by different groups. These series are somewhat related because the quantity of all species ...
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### binary data with no correlation but with p-values equal to real data

For doing parallel analysis in my real data to determine the number of its dimension, i need to simulate binary data matrix with n(number of case) by k(number of items). the p-values of these ...
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### Which type neural networks for time series classification

I would like to use neural networks to classification of time series ( I have some Patterns and I want to adjust input time series to an appropriate class) -ist it possible to do this job with ...
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### Hierarchical clustering of correlation matrix

I have a correlation matrix of 8,854 * 8,854 size. These are Pearson correlation coefficient values in the matrix. I want to perform Hierarchical clustering and create good resolution images like I ...
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### What's wrong with my Kernel algorithm (Kernel SVD)?

I have a user-item matrix $A$ as data input, which is a sparse matrix containing a large number of missing values (as zeros). Each row is a user, and each column is an item. Generally, I am conducting ...
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### Does it make sense to impute year of birth?

This is data cleaning and preparation stage question for me. I apologize if the question is basic, but I am a beginner. I have a dataset of a bit less than 4500 records. This is a survey and ...
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### Discriminant analyses with NIRS in R

I am working with a NIR matrix consistent of 134 rows (samples) and 1529 columns (wavelengths), from which I want to discriminate between two categories (species). I have successfully used the plsda ...
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### Difference between providing probabilities or providing a matrix to the chisq.test function in R

Take this example: ...
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### What kind of test should I do and how many participants do I need for this?

I am designing new functionality in a web application, and I am testing two variations on the navigation to attempt to determine which approach is more successful for our users. I am running the test ...
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### How does Factor analysis explain the covariance and PCA explains the variance?

As I read in Bishop's PRML (here is the clip of that part about FA): According to the highlighted part, factor analysis captures the covariance between variables in the matrix $W$. I wonder HOW? ...
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### What do these statements not follow logically from a 95% CI for the mean?

I've been reading Hoekstra et al's 2014 paper on "Robust misinterpretation of confidence intervals", which I downloaded from Wagenmakers's website. On the penultimate page the following image ...
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### Linear correlation in ANCOVA - necessary?

I've been told by my stats prof that running an ANCOVA relies on a series of assumptions, including that the the dependent variable and the covariate are linearly related. So, in SPSS the first thing ...
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### When is Micro-average Precision = Micro-average Recall

This page describes briefly on micro and macro precision used in computational evaluation: http://rushdishams.blogspot.de/2011/08/micro-and-macro-average-of-precision.html This paper surveys the ...
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### Should I be using a GLMM?

I'm looking at the influence of pollen type on whether a flower sets fruit (i.e., yes or no = 1 or 0). Then looking at number of seeds per fruit (1-6 possible). I was told I should use lmer, however ...
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### Why is the Fisher exact test not always used in the cases where a chi squared is appropriate?

According to Wikipedia: Although in practice [Fisher's exact test] is employed when sample sizes are small, it is valid for all sample sizes. If Fisher's exact test can provide the exact ...
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### Is the distribution of my residuals from a multiple regression normal?

I have a dataset with 17 variables (one numerical, 16 categorical). The numerical variable is the execution time of a process. I'm using one of the categorical variables (100 levels) to predict the ...
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### prediction from incomplete observations

Suppose I have a linear model predicting class-membership from a set of predictors. Now, I am going to classify a new observation which has, however, some predictor values missing. How can I deal with ...
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### How to discretise continuous attributes while implementing the ID3 algorithm?

I am trying to implement the ID3 algorithm on a data set. However, all attributes are continuous and can have values between 1-10. I found that we have to specify the bin intervals for discretization ...
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### Linear model in R - 2 fixed and one random factor

I have performed an experiment on plants and have some trouble analysing the data. For the experiment, we have three replicates (in time), two species and three treatments. In each experiment, we ...
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### Replicator Neural Network, Step-wise function causing same prediction

In my project, one of my objectives is to find outliers in aeronautical engine data and chose to use the Replicator Neural Network to do so and read the following report on it ...
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### Comparing density plots and scoring the combination

I have a set of density plots that contain the distribution of stock prices. Each graph has 5 density plots as follows that shows the distribution of monthly returns based on their ratings - ...
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### AB testing with R [on hold]

I'm new with AB testing and i want to do this with R (free and easy to learn software). Can anyone help with sample code in R for AB testing? Thanks Ariel
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### Forced-choice data possible with confirmatory factor analysis?

I have a task where I ask participants to view a series of images. After viewing each image they are given several options with which they must a forced-choice as to which category the image falls ...
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### How do I estimate the average size of objects

We have a large number of (physical) legal files, and want to get an estimate of the average number of pages per file. I do not know what distribution of pages to expect, the number of pages will ...
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### Probability of Event from a Chi-Square Table

I have the following table: From here I want to answer the following question: P(CHILD CAN RECOG BEG SOUNDS OF WORDS – YES | CHILD RECOGNIZES LETTERS - Most of them). From my understanding of ...
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### Is it valid to use Hartigans' dip test to reject uni-modal null hypothesis with large N?

first time questioner, so please be gentle :) I have two distributions of data from a simulation. By eye, one looks like it might be bimodal, one not. I copy them below A: parameter value 1 : by ...
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### Is the exact value of a 'p-value' meaningless?

I had a discussion with a statistician back in 2009 where he stated that the exact value of a p-value is irrelevant: the only thing that is important is whether it is significant or not. I.e. one ...
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### Significance of the distribution of DNA sequences given a certain nucleotide bias

I have a large sequence of DNA in which every position can have 4 different states, or nucleotides (A,T, G or C). The distribution of the nucleotides in the sequence is non-random and, most ...
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### Converting log odds coefficients to probabilities

Suppose we've ran a logistic regression on some data where all predictors are nominal. With dummy coding the coefficients are ratios of log odds to the reference levels. A colleague claims that we can ...
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### Validity of an iteration for Goodness of fit to a specific application

I am not a statistics expert and would like to check the validity of a test I hav used on a survey results. Survey Results I asked people (n=264) to characterize a panoramic road they chose in 24 ...
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### Optimizing through a Transition matrix, in a non-stepwise manner

I made some sort of a transition matrix of an increase in probability of success, given the allocation of some finite resource (e.g. 4 in this case). At this moment, I consider the allocation in a ...
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### How to test for serial correlation with pooled OLS, FE an RE?

I have pooled data for 3 years, and I have come across the problem of serial correlation. Some books mention the problem of serial correlation when pooling the data. My question is: is it possible to ...
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### How to compare distributions of two groups where each group has multiple observations from a small group?

I have data on continuous measurements (length of time of a behaviour) from two groups of individuals which differ in their phenotype (phenotype A or B, the variable used to group them). Each group is ...
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### Five-number summary and mean

Why was not mean included in the five-number summary, when it was first conceived? What was the motivation of choosing sample minimum & maximum, lower & upper quartile and median?