The normality tag has no wiki summary.
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Transforming data with both negative and positive skew
I am attempting to run a MANCOVA of memory assessment data. My IV has three levels (no memory impairment, moderate memory impairment, and severe memory impairment). The covariate is education. The DV ...
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0answers
44 views
Does the normal approximation get better as a density becomes more peaked?
I have a sequence of densities $f_n(x_n)$, of random variables $X_n$, with means $\mu = 0$ and variances decreasing with $n$:
$$
\sigma^{2}(X_n) = \frac{\sigma^2}{n}.
$$
I am approximating $f_n(X)$ ...
5
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3answers
82 views
Validity of normality assumption in the case of multiple independent data sets with small sample size
Due to limitations in experimental setup, I only have small data sets with n=3. Despite the low df the difference between treated and control is large enough to generate a significant p-value.
The ...
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1answer
62 views
Regression with non-normally distributed residuals
There are several posts on this site talking about the need of normality when interpreting the meaning of the p.value of a linear regression. But not much I think is said about how to deal with ...
5
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3answers
147 views
Normality of dependent variable = normality of residuals?
This issue seems to rear its ugly head all the time, and I'm trying to decapitate it for my own understanding of statistics (and sanity!).
The assumptions of general linear models (t-test, ANOVA, ...
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0answers
25 views
Importance of estimating $\sigma^2$ in linear Statistical model
Statistical model for Complete Randomized design
$y_{ij} = \mu + \tau_i + \epsilon_{ij}$
where, $i$ denotes treatment and $j$ denotes observation.
$i=1,2,...,k\quad and \quad j=1,2,..., n_i$
...
1
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2answers
66 views
P-values from non-normal distribution
I have a collection (approximately 12,000) correlation values. Our correlation analysis does not allow for negative correlations (we correlate with sinusoidal waves, so instead of a negative ...
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1answer
42 views
Importance of the normality assumption in linear statistical models
Statistical model for Complete Randomized design
$y_{ij} = \mu + \tau_i + \epsilon_{ij}$
where, $i$ denotes treatment and $j$ denotes observation.
$i=1,2,...,k\quad and \quad j=1,2,..., n_i$
...
4
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1answer
100 views
How to test for normality of growth disturbances in chemo treatment?
I'm a med student, conducting a retrospective analysis of weight/growth disturbances during chemo treatment.
I wonder, if I should:
assume, that growth is a variable normally distributed across the ...
0
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0answers
26 views
Modelling a skewed, 10-point Satisfaction variable
I am trying to replicate and hopefully improve on an analysis done in a study to find determinants of patient satisfaction after shoulder surgery. Satisfaction is heavily skewed (with over 60% of ...
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2answers
82 views
Should the predictor variables be normally distributed for Poisson glm?
This is probably a really basic question, but it's the first time I've created a model that defines Poisson as its error family.
In setting up my variables to make the model, should I be concerned ...
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2answers
106 views
What is the expected distribution of residuals in a generalized linear model?
I am performing a generalized linear model, where I have to specify a family different from the normal one.
What is the expected distribution of residuals?
For example, should the residuals be ...
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1answer
112 views
What statistical test to use for my questionnaire
I have to do some statistical tests on a questionnaire and don't know which tests i need to do. The questionnaire have been marked as right or wrong and I have about 60 peoples results. The tests i ...
1
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1answer
93 views
All my observations are identical. Use a non-parametric test?
I was doing test for normality on variables and came across one that had identical values for each observation. I can't do a normality test. I plotted the kernel density and it looks normal, however ...
3
votes
1answer
119 views
QQ plot is consistent with normality when subgroups are non-normal
I have read that for a one way ANOVA, you should check that the model residuals are normally distributed. If the variance of each group is homogeneous then this implies that the residuals with each ...
3
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2answers
297 views
QQ plot does not match histogram
I have a histogram, kernel density and a fitted normal distribution of financial log returns, which are transformed into losses (signs are changed), and a normal QQ plot of these data:
The QQ plot ...
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3answers
308 views
Normality assumption and sample size
I know this is a very debated topic, even on this site, but I still couldn't find an answer to my problem.
Recently I am working with large samples (300, 400 and more). For now, I am trying to use ...
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1answer
80 views
Shapiro test and transformation
I am playing with shapiro.test from R and checking for non-normality of error variance.
...
1
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1answer
457 views
Can non normal data be used for factor analysis and multiple regression? If so what is the procedure to justify it?
While I was writing up the analysis in my thesis, I just came across when rechecking my test for normality, that the p-value for most continuous variables was .000, which is less than .05, and it ...
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0answers
169 views
Comparing observed and predicted values across several measurements
As a neuropsychology graduate student with some experience in statistics (I'm usually the guy other psychologists come to with statistics problems after trying it themselves but before seeing a ...
4
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1answer
261 views
Disagreement between normality tests and histogram graphs
My data consist of compaction measurements from 3 different cell types (X,Y, and Z). My goal is to know whether there are "significant" differences between these measurements, so I have tested for:
...
2
votes
1answer
117 views
Which test to use to compare means/medians when one variable has normal distribution and the other does not?
Which 2-tailed test is best to use to compare means/medians when one variable has a normal distribution and the other does not?
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2answers
218 views
Simple question about the asymptotics of estimators
Consider any arbitrary estimator called $\hat{M}$ (e.g., regression coefficient estimator or specific type of correlation estimator, etc) that satisfies the following asymptotic property:
...
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2answers
802 views
Normality of residuals vs sample data; what about t-tests?
An addition to the common confusion about normality testing in inferential statistics for general linear models:
I understand the assumption of normality refers to the residuals in ANOVA and ...
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2answers
499 views
Required conditions for using a t-test
The conditions that I have learned are as follows:
If the sample size less than 15 a t-test is permissible if the sample is roughly symmetric, single peak, and has no outliers.
If the sample size ...
4
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2answers
147 views
Does a zig-zagging residual plot mean that normality has been violated?
I have the following diagnostic plot for my data. Is normality violated, especially given the zig zagging residual plots?
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1answer
508 views
Normality test for anova mixed model in SPSS: I did it in two ways but the results are contrasting
I'm conducting an ANOVA mixed model 2 X 3 (group X condition). Right now, I'm checking for the assumptions of ANOVA, such as normality. I should check indipendently for group and condition, right? I ...
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2answers
368 views
Transformation to increase kurtosis and skewness of normal r.v
I'm working on an algorithm that relies on the fact that observations $Y$s are normally distributed, and I would like to test the robustness of the algorithm to this assumption empirically.
To do ...
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1answer
204 views
Non parametric test on unequal groups and repeated measures data
I am recording animal behaviour in four different conditions but in the same environment. So my data are several values (the number of which vary) representing the frequency of a single behaviour for ...
2
votes
1answer
152 views
Sum of normally distributed variables is a normally distributed variable?
Consider two Wiener processes:
$$
\begin{aligned}
X_a &\sim\mathcal N(0,a) \\
X_{a-b} &\sim\mathcal N(0,a-b)
\end{aligned}
$$
How do I show that:
$$
X_a - X_{a-b} \sim\mathcal N(.,.)
$$
That ...
3
votes
1answer
459 views
Using Student or Wilcoxon test on small sample without information on the normality of distribution
I have three subjects in a qualitative study. I am comparing the features of their writing before and after treatment.
I have no idea about the normality of the distribution since they were selected ...
16
votes
2answers
490 views
Can we see shape of normal curve somewhere in nature?
I do not want to know if some phenomena in nature have normal distribution, but whether we can somewhere see shape of normal curve as we can see it for example in Galton box. See this figure from ...
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1answer
152 views
How robust are multivariate methods to violations of normality? [closed]
In many cases, multivariate methods are used without normality tests.
How are following methods robust if data are not normal?
Principal components analysis
Canonical correlations analysis
Factor ...
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0answers
283 views
Is the Anderson-Darling test or Shapiro-Wilk test more powerful? [closed]
In the testing of normality, how would the 2 compare? Is one significantly better than the other?
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0answers
137 views
Significant factor in 2-way repeated ANOVA becomes not significant in 3-way repeated ANOVA?
My research design is 2(Ear) x 3(Factor A) x 3(Factor B) with repeated measures on all factors. For 2 way-repeated ANOVA, I used laterality index (normal distribution, calculated from % of responses ...
5
votes
2answers
537 views
Normalization vs. scaling
What is the difference between data 'Normalization' and data 'Scaling'? Till now I thought both terms refers to same process but now I realize there is something more that I don't know/understand. ...
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5answers
893 views
How to determine whether data is slightly or extremely non-normally distributed?
I'm a PhD student and doing a research on regression analysis.
My question is how to determine whether the data is slightly, moderately or extremely non-normally distributed?
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1answer
382 views
What kinds of variables should we use the normality test for?
When should we apply normality tests? For which types of the variables should we apply the normality test?
For example dependent variables, independent variables, or control variables, etc?
5
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3answers
251 views
Is there any test for a null hypothesis of non-normality?
I'm currently looking for a test having for null hypothesis that the sample does not come from observing a normally distributed random variable. In other words, I'd like to know if there's a test ...
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2answers
331 views
Skewness, kurtosis and normality of a time series
I have a sample size of $21$ with $496$ observations.Can I presume an approximately normal distribution,and use a $t$-test to compare the difference in means, and difference in various financial ...
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2answers
623 views
Is the sample variance a useful measure for non-normal data?
Does it ever make sense to compare the variance for two sets of data, neither of which are even approximately normal (e.g. bimodal)?
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2answers
2k views
Testing normality assumptions for linear mixed models and mixed (repeated) GLM ANOVA in SPSS
I have a mixed design that includes both repeated (condition) and between (sex and genotype) subjects factors. I would like to assess whether my data meets the normality assumptions for 1) General ...
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0answers
96 views
How to transform a highly leptocurtic distribution to a normal distribution? [closed]
How to transform a leptocurtic distribution into a normal distribution
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2answers
1k views
Can I use a Z-score with skewed and non-normal data?
I've been working with some process cycle time data and scaling using the standard z-score in order to compare between parts of the full cycle time.
Should I use some other transformation since the ...
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3answers
374 views
Does the non-normality matter in using regression for prediction?
Does the non-normality matter in using regression for prediction?
Hi all,
In the QQ plot of the residuals after linear regression, the residuals turned out to be highly non-Gaussian.
Most of the ...
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2answers
290 views
Impact of regression normality-assumption on model comparison & prediction?
This question is a continuation of the discussion here:
How to test the statistical significance for categorical variable in linear regression?
Following Macro's suggestion, I started a new thread.
...
2
votes
1answer
259 views
What happens if you reject normality of residuals when estimating with least square ?
What happens if you reject normality of residuals when estimating with least square ?
Is it too important to have normality on the residuals?
2
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2answers
2k views
Repeated measures ANOVA or Friedman test?
I have a study with 35 subjects who received hypercoagulability testing at 4 different times (before surgery, after, at 1 week, and at 1 month). Each subject has 9 different test of hyper ...
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2answers
3k views
How robust is ANOVA when group sizes are unequal and residuals are not normally distributed?
I understand that there are degrees of trust one can have in the output of ANOVA. However, naturally I want to maximize the amount I can trust my results.
Question: If I have data that violate the ...
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2answers
3k views
How to test for normality in a 2x2 ANOVA?
Study Design: I showed participants some information about sea-level rise, focusing the information in different ways, both in terms of the time-scale and the magnitude of potential rise. Thus I had a ...
