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Questions tagged [normality-assumption]

Many statistical methods assume data are normally distributed. Use this tag for questions about the assumption & testing of normality, or about normality as a *property*. Use [normal-distribution] for questions about the normal distribution per se.

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Tests of normality - qq and Shapiro-Wilk

I am new to the world of stats... My data had a log normal distribution, so transformed by log to get it nearer normal distribution. This is real-world data. From here I want to establish if my data ...
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0answers
72 views

Determine spread of data for non normal distribution

I've collected data from 12 sensors, the sensors are sampled at various times, mounted in one of three orientations and at different temperatures. I'm trying to determine the maximum and minimum ...
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1answer
29 views

Is this a reasonable coarse test for normality?

I'm interested in assessing if the marginalized posterior of a parameter obtained through a Bayesian MCMC process is "more or less normal". I use quotation marks because I'm not trying to asses for ...
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1answer
73 views

Wilcoxon signed-rank test or One Sample t-test?

I conducted a training session and for each of my users, I measured pre and post score (before and after training session). Each of my users (n=28) is measured twice, so my data is paired: Data: <...
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0answers
29 views

normality of independent variables [duplicate]

I am running a generalized linear model. The dependent variable is binomial and independent variables are categorical and continuous variables. My question is : 1. Does the continuous independent ...
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0answers
36 views

Which one of these is correct for linear regression?

Only one of these is supposed to be the correct one for simple linear regression. Which pair of plots would you say has constant variance and normal distribution? I feel like none of them have both ...
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1answer
21 views

checking normality in repeated ANOVA (residuals vs differences)

If I check for the assumptions of a repeated ANOVA (one IV with 3 measurements) I need to check for the normality of differences. From t-test for paired data I know, I calculate the difference and ...
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0answers
13 views

How to verify assumptions of a normal distribution? [duplicate]

I'm working with the iris data.I made a density plot And a qq plot For sepal Length.It looks Approximately normal With the left skew And a flat peak The points In the qq Plot Stay within The ...
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0answers
22 views

Quasi-likelihood Estimation and Linear GEE

I am trying to better understand some of the assumptions of the estimator I am using, which is linear GEE. It is my understand that linear GEE uses quasi-likelihood estimation and so it has weaker ...
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1answer
38 views

Wilcoxon Test - non normality, non equal variances, sample size not the same

I know there are already a lot of posts out there, but I couldn't find this exact combination in any of them. Comparing two samples (Prices associated with men and with women), but I have neither the ...
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0answers
10 views

Normality of residuals in a VAR model

If I have these differents results for the normal distribution of the residuals, I should consider them normal or not? $JB JB-Test (multivariate) data: ...
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1answer
16 views

Generalized Estimation Equations 3 levels

I have a dataset where students are nested within classes, and classes are nested within schools. I am interested in evaluating the effects of a treatment delivered at the student level. The response ...
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1answer
33 views

Correct normalty preconditions for the t-Test

Lately, I have had several data sets at hand that I want to use to study the effect of a treatment. The data is in the form of: \begin{array} {|c|c|c|c|} \hline \text{Participant} \quad & \...
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0answers
11 views

Pros and cons of using a ratio vs a difference score to compare group means

I plan to collect data from 2 groups of people (Gp_1 and Gp_2), some few hundred people in all. Each person will respond to 100 yes/no tasks, each response yielding both accuracy and reaction time ...
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0answers
41 views

How do I interpret the p-value from a Shapiro-Francia Test?

I have a situation where I have more than 50 samples in a given set of inputs and I cannot use the Shapiro-Wilk test as I don't have the numbers for the pyramid for $n>50$. I was then asked to use ...
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0answers
10 views

testing normality in repeated measures

For my bachelor thesis I collected data. I have a within subjects-factor (2 levels) and a covariate. I am using SPSS. When I want to check normality of my sample, I know I need to do that with the ...
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1answer
54 views

Comparing linear mixed effects models using ANOVA - underlying assumptions

I am trying to analyze some CPUE (Catch Per Unit Effort) data for a fisheries related analysis with lme models in R. So I have total of six models that are tested in groups of two to determine the ...
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1answer
59 views

Assumptions of linear mixed model not met

I have a repeated measures dataset with which I'm testing if individuals are consistent in their boldness scores (continuous variable) over time (trials). Towards this, I generated linear mixed ...
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1answer
20 views

Is normality test a must for SARIMA forecasting?

My data is not normal and I have tried Box-Cox transformation, yet after Box-Cox transformation, it still fails under the Kolmogorov-Smirnov test, so can I skip the normality transformation and use ...
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0answers
60 views

“This is a lower bound of true significance.” in Kolmogorov-Smirnov. WHat does it mean?

I' conducting a Normality Test for a numerical variable (level of a substance in blood) in a sample of 368 subjects. As I will need to run analysis of covariance (ANCOVA) and partial correlations, I ...
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0answers
68 views

Normality assumption

Can you tell me what should be normally distributed in t test and ANOVA? Almost every book about statistics contain an information that in t test the dependent variable should be normallly ...
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0answers
25 views

Can the Berry-Esseen theorem tell us whether acceptable inference may be achieved by parametric tests?

I refer in particular to such choices: 1) t-test (or its generalizations: ANOVA or Hotelling's $t^2$) vs its non-parametric alternatives (e.g., U Mann-Whitney test and its generalizations); 2) Pearson'...
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0answers
46 views

How do I adress non-normal residuals in gam/gamm

I am trying to fit a GAM where the outcome (y) depends on 2 continuous variables (v1 and v2) and 4 categorical variables (v3-v6). $$M1 <- \mathrm{gam} (y \sim s(v1) + s(v2) + v3 + v4+ v5+ v6)$$ ...
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1answer
23 views

Meta analysis Cliff's delta

Cliff's delta is an effect size typically recommended whenever the normality and homogeneity assumption do not hold---and thus, Cohen's d cannot be used. However, I cannot see any reference ...
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0answers
44 views

Censored Regression/Tobit: coefficients decrease drastically when non-normal distribution is assumed

I am trying to use a Tobit/ censored regression model to estimate the effect of a certain political condition on a tax. Our dependent variable is zero-inflated because in most of our observations this ...
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2answers
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When checking for normality, would you prefer numerical methods or graphical methods?

I would like to know what is your preferred way to decide when the deviation from the normal distribution is relevant. when saying "relevant", I not saying necessarily "statistically significant". ...
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22 views

Why linear regression needs normality assumption [duplicate]

In this page, it mentions that linear regression requires residuals to be normally distributed. Why we need this assumption, and what will happen if this assumption does not satisfy?
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1answer
60 views

Normal distribution test ( D'Agostino and Pearson's) in Scipy - results don't make sense

I want to test if an array of numbers is approximately normally distributed. ...
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0answers
36 views

Relationship between t-test and normality

I haven't been able to find a definitive answer to my question in previous topics, so I'll post it here. I am sure this is a trivial question for many, but I do not have ny formal training in ...
3
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1answer
75 views

What should I do after multiple imputation in the data?

I have a data set with missing observations. I used VIM package in R for imputation. After imputation, I will try to run a ...
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0answers
24 views

Asymptotic consistency and normality

I need help getting the following problem Let $X_1,..,X_n$ be independent $N(\mu,1)$-distributed random variables. Define $\hat{\theta_n}$ as the point of minimum of $\sum_{i=1}^n(X_i-\theta)^4$...
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1answer
50 views

Paired t-test of a non-normal data with sample size 66

I have a paired data with n=66. I understand that normality condition of differences data should be met to conduct a paired t-test. However, from the histogram, it doesn't appear so normal. The p-...
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1answer
64 views

A powerful test for any distribution

Simulations by Razali et al (2011) showed that the Shapiro-Wilk test of normality provided the most power (at a fixed significance level) when compared to Anderson-Darling, Kolmogorov-Smirnov, etc. My ...
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25 views

Multivariate Normality ANOVA test

ANOVA requires multivariate normality as a key assumption. I understand that this assumption can be tested in MANOVA using the Mardia test, Royston test, and Henze-Zirkler test. This can be ...
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0answers
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Normality test and parametric vs non parametric tests

I'm pretty new in statistical testing and I'd like some help in what I'm doing. I have two groups of scores from 0-100. Group A has 10 values and group B has 70 values. Initially I'm testing normality ...
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1answer
21 views

Instrumental variables with non-normal endogenous

Does it make sense to conduct an instrumental variables model where the endogenous variable of interest is continuous but not normally distributed? I know for normal regression purposes, there is no ...
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0answers
29 views

What is the difference between Shapiro-Francia and Shapiro-Wilk tests of normality?

I apologize for this easy/basic question. Please forgive me if this is a duplicate question. I've been researching the Shapiro-Wilk test (1965) of normality for a project. In doing so, I found an ...
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0answers
62 views

Point Biserial Correlation

In terms of testing the assumption of normality for a Point Biserial Correlation; I have a dichotomous variable (1 = microsleep, 0 = no microsleep) and a continuous variable (number of driving events ...
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0answers
17 views

Normality assumption for t/z tests

I have what is probably a silly question regarding normality assumptions for t/Z tests. As I understand, t/z tests require that sample data was obtained from populations following a normal ...
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2answers
41 views

Checking the Assumptions for T-tests

I am in the porcess of checking the assumptions of some data in order to perform a T-Test and had a few questions about how they should be set up. I was able to find the assumptions for a T-test here....
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0answers
32 views

Relevance of residual normally distributed residuals in nonlinear regression

I have a mathematical equation, based on physics, that requires estimating several parameters via nonlinear regression. I have conducted such nonlinear regression estimation with a dataset of 1100 ...
2
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2answers
74 views

Normality within cells and non normality of residuals

In testing for the normality assumption for 2 way ANOVA, the question whether normality within cells implies normality of residuals has been asked before and I have found so far different answers from ...
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1answer
111 views

Normality test and Outlier detection [duplicate]

In this question, I would like to ask two things: outlier detection normality test Details are as follows: I need to detect and remove outliers in my data. Before doing that, I want to test if my ...
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0answers
42 views

Normality of residuals in Boosting

I'm very new to the machine learning field and am trying to apply boosting to my models to improve its predictive ability. I was doing some reading into tree-based approaches and was wondering whether ...
3
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1answer
135 views

What is the CDF of the Shapiro-Wilk $W$ statistic?

I am trying to understand how Shapiro-Wilk test functions. So far, I have come across the following links: How are the values in the Shapiro Wilk weight table calculated? which explains how to ...
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0answers
25 views

Assessing Normality in ANOVA - Dependent Variables or the Studentized Residuals? [duplicate]

I ran an between-subjects repeated measures (2*2*2) ANOVA in SPSS using GLM. One of my dependent variables didn't meet the Test of Normality (Shapiro-Wilk p = 0.047) according to the table. The Q-Q ...
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0answers
45 views

When is testing for normality necessary for machine learning with Big Data?

When is testing for normality necessary for machine learning with Big Data? Please give examples or counter examples.
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2answers
36 views

How to test difference in means (due to outliers) when the distribution is not normal?

Background I have two conditions: A and B, with around 400 measurement points for each condition. The most common result for both conditions is around '700', and neither one has a result below '600'....
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1answer
196 views

Paired sample t test normality violation

I would like to conduct a paired sample t test and thus, I'm checking for the assumptions of normality. Upon conducting normality testing, each group scores were found to be normally distributed ...
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1answer
84 views

Does assumption of normality of each mixture components implies that each margins is normal

I just would like to understand some information about the joint normality and the margins. I read that the normal joint distribution almost always implies that the univariate margins are all normal. ...