# Tagged Questions

Refers to the normal distribution, the Gaussian continuous probability distribution.

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### Which normality? [closed]

One of the assumption of t-test is normality. Which I want to check using the kurtosis and skewness on spss. But do we check normality for the dependant variable or for each group on the dependant ...
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### Is the dataset Normally distributed?

So based from this link http://www.simafore.com/blog/bid/107702/2-ways-of-using-Naive-Bayes-classification-for-numeric-attributes I began to realize it might be a good idea to compute the pdf of ...
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### Is there skew/kurtosis on this plot?

I'm doing a stats assignment and for one of the questions I need to make a judgement of whether there is skew and kurtosis from a p-p plot in SPSS. I've been over the lecture, and we were told to look ...
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### Is this a 2 fixed unbalanced ANOVA? How can be tested normality and homoscedasticity?

I need to know if my biological experiments show discrepancies between the condition used. In my experiments I have 2 fixed conditions: type of substrate (2 types) and chemical added (1 control + 3 ...
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### Four standard deviation rules in relation to normality testing

Within our department we were given a set of guidelines for how we might decide whether to proceed with parametric or non-parametric testing (for continuous data). These were drawn up in consultation ...
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### is normality of raw data ever important? [duplicate]

http://www2.psychology.uiowa.edu/faculty/mordkoff/GradStats/part%201/I.07%20normal.pdf https://pubs.er.usgs.gov/publication/5224239 The two links above seem to me to claim the opposite. Is normality ...
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### Normality testing for t test assumption

I am confused in what variables to input for normality, whether IV and DV go in together or seperately? I have an IV (high anxiety and low anxiety) and 6 dependant variables. When i check for ...
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### Compare several means with different sample sizes (greater than 2)

I have seven different groups with different sample sizes and variances and I want to compare the means of their data. I'm not very informed in statistics, so could anyone help me out here? I've only ...
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### Normality test in panel data analysis

Is it necessary to perform normality test such as Jarque Bera, Kolmogrove etc in panel data analysis? Assume I have 3000 obervation..Since the observation size is big, can I just assume that the data ...
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### Is there an explanation for why there are so many natural phenomena that follow normal distribution?

I think this is a fascinating topic and I do not fully understand it. What law of physics makes so that so many natural phenomena have normal distribution? It would seem more intuitive that they would ...
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### Pair t-test or Welch's t-test on a non-normal data series?

I have rainfall time series of 85 points from 1901-2013. I want to check whether the mean rainfall during 1901-1970 is significantly different than that of the mean rainfall during 1971-2013. Here, ...
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### Normality and homogeneity test for small sample size

I would like to inquire if testing for normality and homogeneity is still necessary for my data from an experimental where trials were done only three times. Or can I directly perform ANOVA and ...
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### ANOVA and Kruskal-Wallis Test in One Study

I am currently analyzing the results of my study which deals with several dependent variables. I have tested the data for normality and homogeneity and all but one passed the assumptions for ANOVA. ...
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### How do you know if a distribution is normal? [duplicate]

Say I have a data set, how do I know if the distribution is normal or not? If not, how can I tell what type of distribution it is? Are there some tests for identifying distribution types?
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### OLS Regression : Efficiency of the estimator of the variance of the residuals under the assumption of normality

My question is probably already answered somewhere but I did not find it. In the standard linear regression model under the assumption that residuals are normally distributed, we have a result ...
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### Mixed effect models and data that has floor effects (and is proportions)(sort of)

I've been asked to analyze some data using linear mixed effect models (via lmer in R). However, I'm not sure this is the appropriate approach to the data. I am going to provide some background detail ...
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### Help accounting for heteroskedasticity in a Revenue~GHG model with out Log transforming

Context I work for a sustainability charity and I've been developing regression model to estimate the Greenhouse Gas Emissions of public companies based on reported data (both financial and ...
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### Statistical tests for uneven groups

We performed an experiment (usability testing), which had two independent samples, one group with 5 participants and the other with 24. Based on the analysis of the distribution of the data ...
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### QQplot vs BoxPlot

I am having trouble reading my qqplots. I want to asses normality graphically before I carry out a test like shaparo test(my sample is small (<50)) . So I have two question is the qqplot better ...
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### Shapiro-Wilk Normality Test

I am very new to statistics and need help interpreting a Shapiro-Wilk Normality Test. I have 2 data sets, one caled TMS and the other Placebo. I ran the test and got this back from R: ...
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### Residual non-normality and prediction intervals

Normal residuals are generally understood to be necessary for valid prediction intervals in OLS regression -- but I've never seen a definitive guidance on just how much non-normality can be tolerated, ...
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### Role of Central Limit Theorem in one-way ANOVA

Background: It has been shown and widely referenced (applets even exist, etc.) that for even a highly-skewed numeric variable, a sample size of $n\ge{}$30 is often "large enough" for the Central Limit ...
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### Comparing two means (randomized experiment)

I'm kind of new to science :) for my thesis I worked on some study on health related behavior. There was an intervention for the experimental group and none for the control. So that seems to be a nice ...
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### What type of data would have non-normal errors?

I'm trying to understand the assumptions for an OLS model. I get that the error term should be normally distributed if we want easy-to-calculate confidence intervals for our coefficient estimates. ...
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### Normality test on logarithmic data

I had to take the log of my data, and now I want to test for normality of my data. Can I use the standard normality tests or do I have to use some special test? I used the ...
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### Which option is better: checking 1) normality of data or 2) normality of residuals for Anova?

Two types - data and residuals - are indicated for checking normality for anova.
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### Repeated-measures ANOVA vs. Friedman-Test (normality?)

I've four (paired) datasets consisting of 43 rows and want to check if there is statistically significant change between those groups. To see whether a repeated-measure anova or it's non-parametric ...
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### Testing for normality - low p-value

I've a dataset with about 12k values given. It looks like this: When I try to test for normality I get following results, where p-value is extremely slow: ...
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### Can I run a normality test on percentages?

First I'll start out with some background information. I conducted image analysis on 200 images to identify the area % of 7 different phases. I then averaged the results from each of for the 200 ...
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### Do I have to test normality for both groups when comparing from a single population?

This should be an easy one. I'm a novice when it comes to statistics and English isn't my first language so bear with me. I have one population that numbers about 700. Of these 700, 25 are of special ...
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### The Friedman's test

I have conducted research on the impact of toys on reading comprehension among primary school children. I have tested the strategy with three types of English language learners; good, intermediate and ...
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### Test for Conditional Normality?

On page 357 of this article, it states: We assume that the treatment (or its transformation) has a normal distribution conditional on the covariates. How would I go about determining if a ...
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### Asymptotic normality for nonsmooth objective functions

Assume that $f ({\bf x}; \theta): \mathbb{R}^p \times \Theta \to \mathbb{R}$, where ${\bf x}$ is the vector of inputs (with some distribution) and $\theta$ is the vector of parameters. Also, assume ...
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### Correction for non-normal residuals where residuals are not correlated or show heteroskedasticity

I know the implication of non-normal residuals is uncertain statistical tests because the SEs are inefficient. Can I apply the Newey-West to calculate standardized Standard Errors for an OLS ...
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### How incorrect is a regression model when assumptions are not met?

When fitting a regression model, what happens if the assumptions of the outputs are not met, specifically: What happens if the residuals are not homoscedastic? If the residuals show an increasing or ...
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### Reference for standardizing non-normal variables for comparison

I need a reference to justify standardizing non-normal data. I will first explain what I am doing. I had a survey with ~30 questions. The answers to the questions were numeric; the range of possible ...
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### Testing of pre-post differences for multiple conditions and non-normal distributed data

I have pre- and post-test behavioral data (response times) of 9 people (1 group). The task which people performed was divided into 5 sections which differed in the stimulus type. So I have variables: ...
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### Normality test: descriptive vs inferential

REVISED ON REQUEST: Is normality test conducted to check sample normality or population normality, most of the times normality test is required to validate the assumptions of parametric test i.e. the ...
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### Good Multivariate normality coefficient but suspicious univariate indices in AMOS, what to do?

We are conducting a research with 15 observed variables. Before starting a SEM model based on our data-set (of 531 cases), we tried to check for uni-variate & multivariate normality problems. The ...
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### Assess moments & normality of data from histogram

I have produced a histogram of my actual data; then I plotted its normalized graph on the same plot. How can I compare the first four moments (mean, variance, skewness and kurtosis) of the actual data ...
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### Normality of conditional pdf(s) does not imply BVN

Show by means of example that the normality of conditional pdf(s) does not imply that the bivariate density is normal. I know of the following example that if $\ U,V,W$ and $\ T$ are independently ...
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### Is a bivariate normal distribution necessary for a significance test / t test of a correlation coefficient? [duplicate]

I am a student and I currently carry out coursework. I am investigating whether there is a correlation between change in unemployment rate and change in GDP (Okun's law) for Germany. As part of this ...
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### Kernel density estimation to check whether distribution is lognormal

I am using the kdens package in Stata to estimate the kernel density of some economic data. I would like to check if my data could possibly come from a normal distribution. My question is about the ...
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### normality tests of residuals in repeated measures design

I have a repeated measures design where I have plotted the residuals and the majority of my data are not normally distributed. As I understand it, repeated measures designs are quite robust when using ...
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### Why do statisticians prove asymptotic normality?

In many statistics papers, authors suggest a new data analysis methodology and prove its properties such as consistency or asymptotic normality. I think it's a kind of tradition or custom. I ...
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### Are two asymptotic values enough to fail the test of normality?

Looking at this post I started to wonder about the gestalt interpretation of the QQ plots generated by qqnorm in R. Here's the plot to avoid having to go to the ...
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### Normality test with large data set

I think I am dealing with this issue http://www.r-bloggers.com/normality-tests-don%E2%80%99t-do-what-you-think-they-do/ I have large data set(10k data points) that slightly diverges from normal, and ...
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### Why is Normal Distribution required to perform an independent t-test? [duplicate]

I've been told many times that, when carrying an independent t-test, I have to determine whether my dependent variable (DP) is normally distributed on boths levels of my independent variable (IV). On ...
Let's see the comparison below Asymptotic normality is given by CLT for both cases. In MLE case, a variance of $\hat{\theta}$ is in distribution as $\frac{1}{I(\theta)}$, but in OLS case ...