Questions tagged [qq-plot]

A qq plot (or quantile quantile plot) is a scatterplot of the quantiles of two distributions. QQ plots are useful for comparing distributions.

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QQ plot result doesn't correspond to normality test [duplicate]

I am considering an ANOVA model for such data: ...
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QQ Plot help meaning [duplicate]

How can I interpret the following QQ Plot? Can you explain it for example for the point 20 and 12?
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How to interpret a point in the qq plot? [duplicate]

I am trying to understand the Q–Q plot. Suppose I create a sample according a exponential distribution ...
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What is y-axis on QQ plot?

I have a simple question about QQ plots in simple linear regression, but I am a bit confused about the plot: depending where i look the y axis is different: it can be either 1) residuals 2) ...
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Random Effect in twin study of both MZ and DZ twin pairs

I am a bit stuck with the use of linear mixed models and its random effects in a twin study analysis. What I have is microbiome data for twins that are from MZ twin pairs and DZ twin pairs, so no ...
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qqplot result and normality for ANOVA

The independent variable is promotion, and it is assigned to 3 groups. The dependent variable is sales revenue. I have 172 observations of sales revenue for promotion group 1, 188 for group 2 and 188 ...
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Interpret the QQ plot of multilevel logistic regression

I'm doing multilevel logistic model in R. To check the model diagnostic, I draw the following QQ plot. But it seems hard to say in which way it skews. I'm a bit confused what conclusion can we draw ...
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1 answer
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Dealing with bimodal residuals

I want to run linear models to understand the effect of single predictors on an outcome. This is quite straightforward, but I am facing a situation where my residuals appear to be bimodal. I can't ...
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How to use R to plot $f_n(\theta)$ based on following samples? [closed]

For iid random samples $X_1,\dots, X_n$ with mean value $E X_1=\theta$, take $X_{(1)}=\min \{X_1,\dots, X_n\}$ and $X_{(n)}=\max \{X_1,\dots, X_n\}$. If $\lambda$ solves equation $$ \sum_{i=1}^n \frac{...
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How to interpret this qqplot?

I wonder how to interpret this plot. I know, in general, that a qqplot depicts the quantiles of the data against the quantiles of the normal function. How do I apply this to this plot?
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Equally spaced points on x-axis in exponential distribution probplot/qqplot

I have a sample distribution and want to check if it is exponential. For that I am trying a qqplot and probplot. ...
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Weighting the calculation of sample quantiles

Is there a general method for determining what the weights are when calculating sample quantiles? Specifically, how do you do this in the case of repeat measurements -- can (should) you take the ...
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Quantile estimation for discrete data

For quantile estimation for data coming from a discrete distribution, do you have to use one of the quantile estimators R-1, R-2, or R-3? For example, R-3 uses the nearest even order statistic after ...
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How do you do a qq plot in R for a discrete Weibull distribution?

To do this, I think I need to calculate the inverse CDF, but I have learned here that the discrete Weibull (type I) as given by: $$F_{I}(x)=1-\exp\left[-\left(\frac{x+1}{\alpha}\right)^\beta \right]$$ ...
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1 answer
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Quantiles from histogram?

Is it possible to calculate quantiles from a probability histogram P(x) rather than from the data x itself? I have an unbinned histogram for discrete random variables, and I'm wondering if I can ...
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1 answer
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Finding the region in sample data that fits to a known distribution

Given sample data that have a known probability distribution for large x and an unknown probability distribution for small x, what is the best way to find the value of x where the known probability ...
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3 answers
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Why are raw data analyzed rather than the probability distribution the data are sampled from?

QQ plots and other methods to analyze probability distributions seem to require input of the raw data itself rather than the probability distribution that the data sample. Why is that the case, even ...
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QQ-Plot for data on skewed t distribution in R

I have been trying to plot my financial data against a skewed t distribution in a QQ plot in R for some time now, but I have only managed to do it against a symmetric t distribution so far. I use the ...
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1 vote
1 answer
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Interpreting QQ plot (Normal vs Heavy-tailed)

I'm having some trouble interpreting the shape of this distribution. It is a distribution of price differences between an estimate and actual price. There are 219 points. I'm not sure if I can call it ...
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1 answer
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Different results for residual normal distribution between Jarque-Bera test and Q-Q Plot

I am trying to test for normality of residuals using 2 different ways: Using Jarque-Bera test Q-Q Plot I can see different results, for the JB test the value is 19.9553 with a probability of 0.00005....
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Some diagnostic plots: how to interprete this QQplot with heavy tail according residual autocorrelation [duplicate]

I am cheking my lm model with QQ plot and Residues. it seems be heavy-tailed. Is this interpretation correct? How are these plots to be interpreted? Any suggestions would be highly appreciated.
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1 answer
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Must the QQ-plot line be at a 45 degree angle? [duplicate]

A qq-plot for a regression I ran looks like this: My understanding was always that there should be a 45 degree angle, i.e. from approx bottom left to top right corner. Must that really be the case? ...
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2 answers
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How to check for normal distribution in R if you have more than 5000 values (alternatives to shapiro-wilk or ks-test)?

I m trying to check my data normal_data for normal distribution in R. I always used shapiro.test or ...
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What mean points all over Q-Q plot?

I printed the Q-Q plot for my data and it came out like this. Obviously, the data is not following the normal distribution, but what else can we say about the data basing on such output?
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Is there a "Weibullity" test?

There exist several tests of normality. This is, tests for checking if a data set is normally distributed. Is there a "Test of Weibullity"? This is, a test for checking if a data set is ...
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1 vote
1 answer
398 views

What does a flat qq-plot mean?

I was looking at some newly gathered data comprised of reading times measured in ms and I decided to plot the observed values prior to cleaning the data to see to what extent the data is normally ...
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Fitting distributions to censored and uncensored data in R

I need to fit lognormal, Pareto, and generalized Pareto distributions to some empirical data that is a combination of censored and uncensored data. I tried using the function ...
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1 answer
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Looking for normality with histogram and QQ

I have some data that looks similar to the example picture below. The difference is that the peaks on the histogram are further above the distribution line and the tails of the Q-Q plot are heavier ...
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6 votes
1 answer
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Best way to construct a QQ-plot

I want to assess the normality of a dataset (which is log-normally distributed data transformed back to normal) using a Q-Q plot. I stumbled on the fact that there are many ways to build such a plot, ...
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3 votes
1 answer
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Interpreting QQplot

This is a question about how to understand qqplot results. To present the problem, I have a set of 54 measurements from archaeological objects. I calculated both kurtosis and skewness with the Moments ...
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5 votes
1 answer
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Can we use the R square of the q-q plot as the Shapiro Wilk effect size?

The q-q plot is a nice way to see a deviation from the normality. But this is only "eye estimation" Can the R square of the linear regression be the effect size of the deviation from the ...
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1 vote
2 answers
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Q-Q Plot Comparison [closed]

Hello these are both Q-Q plots I have created for my models. The top is a Q-Q plot for an old model, and the bottom is a Q-Q plot for an improved new model. What is the difference between the two ...
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At chi squared qq plot, shouldn't normal samples be placed diagonally with chi-square?

I tried to draw a chi-square qq plot from sample following a bivariate normal distribution. This is my code: ...
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2 votes
1 answer
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Can I still do a one-way ANOVA with underdispersed data?

My data passes the Levene test for homogeneity of variance, but fails the Shapiro-wilks test by a lot [p=1.818e-12], I have 549 observations [absorbance values] from 3 different temperatures. The Q-Q ...
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0 votes
1 answer
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qqplot of inverse Gaussian distribution in r

I wanted to draw a qqline of my data with the inverse Gaussian distribution, however, the line printed does not seem to be right, for details see the picture attached. someone help me please thanks
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0 votes
1 answer
60 views

What test is appropriate for normality?

I would like to check the normality of nearly 400 individuals. I tried many test like Shapiro test, etc...but, not much expected results. So, Could you please tell me what is the appropriate test for ...
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0 votes
1 answer
296 views

Shapiro-Wilk Test vs Boxplots

I have a question with regards to normality of the data. When I do a shapiro-Wilk test I'm getting a p>0.05 (which means that my data is normally distributed). However, when I plot my data using ...
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3 votes
0 answers
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Dealing with leptokurtosis, should I be worried?

I modeled a mixed model with the lmer library in Rstudio and my residues don't seem normal, with a Shapiro-Wilk test p-value <0.0001 and a QQ-plot that looks heavy tailed. I then plotted the ...
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0 votes
0 answers
322 views

R: remove outliers from boxplot or qq-plot?

I am running a regression model, and I need to delete outliers. However, when I ran a boxplot, it asked me to delete at least 100 datas (I only have 700 datas in total). Both y and x variables are ...
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1 vote
0 answers
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Interpreting confidece interval qqPlot of a multiple regression

Hello I have a problem when trying to interpret a multiple linear regression graph and the qqPlot() function. I'm trying to do this with an example data of at least 28.000 obs. I used R to make the ...
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A big issue for nonnormal distribution of residuals in this case?

I use both histogram and Normal Q-Q plot for the residuals of my regression. I would say that the histogram shows somewhat symmetric and the mean is around 0. On a Q-Q plot, the points seem to fall ...
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0 answers
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multiple regression model - interpreting graphs for the fit [duplicate]

I have the following r code. I created a multiple linear regression model on a math_and_alcohol dataset. I can see in the summary of the model that the r-squared is .8279 which means the model ...
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4 votes
0 answers
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residuals are not normally distributed - what now?

I am running a mixed effects model and the residuals/qq plot look terrible. I have also ran a glmer model to check if the results change when using a better model and they lead very much to the same ...
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1 vote
0 answers
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Is there a decisive way of accepting/rejecting homogeneity and normality tests?

While learning about regression, I was told as a prerequisite for significance testing you must test for normality and homogeneity. I can do this by plotting Q-Q plots and residuals to fitted values, ...
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3 votes
1 answer
54 views

Is it poissble to quantify the results of a Q-Q plot? (and does it make sense to?)

I have recently come across Q-Q plots and their usefulness in regards to visually inspecting whether a data sample follows a particular distribution. Is there a way of quantifying the results of a Q-Q ...
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2 votes
0 answers
294 views

interpretation of the slope of the qq plot

Reading https://seankross.com/2016/02/29/A-Q-Q-Plot-Dissection-Kit.html (one of the first results about interpreting qq plots) I find myself confused about the slope of the qq plot It is my ...
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2 votes
0 answers
107 views

Binary $Y$ but normal residuals?

In regression, it is abundantly clear that $Y$ can be non-normal while the residuals $\epsilon = Y - \beta_0 - \beta_1 X$ are normal. But can $Y$ be binary when the $\epsilon$ are normally distributed?...
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4 votes
2 answers
856 views

Can non-linear dependence be detected between two variables by regressing them?

Linear regression is meant for linear relationships right, so, if I don't trust linear correlation and want to find out if random variables $y$ and $x$ have a non-linear relationship, can't I just ...
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2 votes
0 answers
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Issues with random effects: what to do when analyzing sites with all 0's or all 1's?

I'm analyzing data that looks at the survival of artificial bird nests under different treatments. These nests were grouped across 80 sites, with 6 nests per site. I have created a binomial model on R ...
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9 votes
3 answers
13k views

How to interpret this shape of QQ plot of standardized residuals?

I am running linear regression for a continuous variable (not standardized) with age and 2 other numeric continuous variables (not standardized), 2 categorical variables with 3 levels each and 1 ...
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