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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26 views

2-sample t-test VS Mann-Whitney test - one group is not normally distributed

I'm comparing two groups of data. One group has a sample size of 30, and the other a sample size of 45. The second group is not normally distributed, and thus this violates the normality assumption. ...
11 views

Check normality assumption for a 3 by 2 within-between (mixed) ANOVA

I'm conducting a 3 (within) x 2 (between) mixed ANOVA in SPSS and would like to assess whether my data satisfy normality assumptions. Do to so, I have saved the unstandardized residuals from the ...
18 views

Coverage of confidence interval of non-normal distribution

I know that when a sample is non-normally distributed, I need to use non-parametric alternatives, as when the assumptions of parametric methods are violated, its coverage will decrease. Let's say ...
16 views

How to understand the normality assumption of the errors?

We know one of the Gauss-Markov conditions is $\mathrm{Cor}[e_i, e_j]=0$ for $i \neq j.$ Also, we assume the errors are normally distributed, i.e., $e_i \sim \mathcal{N}(0, \sigma^2).$ My teacher ...
22 views

Variable does not follow a normal distribution, can I trust its p-value from a Unit Root test? [closed]

If a variable does not follow a normal distribution, I should not trust in the p-value from a Unit Root test, right?
64 views

GLMM for continuous response in $[0, 1]$

I am looking into GLMMs because my linear model residuals' plot has a weird pattern (some residuals form a diagonal pattern), and a lack of normality was also confirmed with a Shapiro test. ...
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Does a PCoA (or MDS) assume normality of the variables behind distances?

More precisely, if I conduct a cmdscale (classical multidimensional scaling) on an Euclidean distance matrix by considering $n$ observations of $p$ variables i.e. \$D_{ij}=\sqrt{ \sum_p (x_{ip} - x_{...
46 views

Checking normality for a t-test

I am following stats courses at the moment and I am a bit confused about performing a t-test. I know that a t-test assumes normality and enough sample size. In the course I am attending, the ...
55 views

How safe is that to ignore homogeneity of variances test and continue with Post hoc test?

I want to know if average time people spending on their favourite social media are statistically different. I consider here five groups, and the total number of participant is 700 persons. I have ...
38 views

Is it correct to do stats in log transformed metabolomics data?

I have a dataset from targeted metabolomics analysis, the units I am working with are ng/ml[creatinine] (I use creatinine concentration to normalize the data since the samples are urine and can have ...
46 views

Testing normality for residuals

I have an Randomized block design experimental set up and a data consisting of species abundance. I would like to test for normality of the residuals, as in unexplained variance due tot the blocks. I ...
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Can one use parametric tests (e.g. ANOVA) if part of the data/ variables meet normality criteria but others not?

I want to compare differences between two groups(n1=26, n2=18) regarding their performance on different tests but also explore how each group performed on different times of these tests. I am ...
424 views

Dealing with heteroscedasticity and non-normality in a mixed model

I am trying to fit a mixed model (person as random effect) on data which has heteroscedasticity and non-normality. I log-transformed the Y-variable but it did not fix the problem. Normality and ...
24 views

Panel data regression - normality and testing issues

Im doing an event study using panel data regression with abnormal returns for 15 events and 38 stocks as the dependent variable, and different multiples (P/E, B/M etc.) as the independent variable. I ...
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How to choose cut off for winsorization/ flooring- capping? What is the impact of variable distribution on the decision

To perform logistic regression I wish to winsorize outliers in independent/ explanatory variables by flooring and capping independent variables. Can you suggest how I should choose cut-off for ...
19 views

Usefulness of rejecting null in normality test [duplicate]

In this Stackoverflow answer Ian Fellows said, Normality tests don't do what most think they do. Shapiro's test, Anderson Darling, and others are null hypothesis tests AGAINST the the ...
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Are these the correct residuals to test for normality for a within-subjects 2-way anova?

I have data of an experiment where subjects performed a task under 4 conditions (A1B1, A1B2, A2B1, A2B2, where A1/A2 are the levels of factor IV1 and B1/B2 those of ...
36 views

Is checking for normality and linearity in this way correct?

Testing normality using a significance test is a little bit useless especially when you have a large sample size So, recently I found a way to test normality by running a fake regression using ...