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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17 views

QQ plot looks like gaussian, but kolmogorov smirnov test suggests to reject null hypothesis

I am testing my estimation of maximum likelihood parameters using the normal equation for gaussian generalised linear model. I am trying 3 covariates and 4 parameters (i.e. includingintercept) and in ...
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Interpreting Shapiro-Wilk-Test, skewness and kurtosis in the light of a qqplot and histogram

Currently I'm trying to find out if my data with n=11 follows a normal distribution to decide how I process further. To find this out I use the Shapiro-Wilk-Test which gives me p < 0.05 and thus I ...
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28 views

How to superimpose rotated histograms and/or bell curves on the side of a scatterplot [closed]

Does anyone know how to superimpose a rotated density plot and/or histogram on the right-hand-side of a scatter plot? I'm agnostic about what software could be used to do this...ideally it would be <...
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How can I check if my data-frame is normally distributed in R?

I have a data frame with 7 columns that holds numerical and integer values where some columns, even though numerical, are binary values (e.g. a dummy variable for sex; $0=\text{male}$, $1=\text{female}...
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Betareg Half-Normal Plot “Error in chol.default(K)”

I am running into a very odd problem when calling plot.betareg (betareg package in R) to produce half normal plots: Sometimes I receive a plot, sometimes I get for the same model with the same data an ...
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66 views

How to modify a linear regression model when the variance is not constant?

I have a very basic question. I have a model that has residuals that don't follow a constant variance pattern, but rather it has a clustered pattern (see the figure attached). Any idea of how to deal ...
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49 views

How to read the mean and standard deviation from a Q-Q plot

I wrote the following code in R to make the following qqplot. In a question I am asked to read the mean and standard deviation of the sample off the plot. I see ...
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44 views

Interpreting qq plot from ARIMA residuals

Im trying to undestand this qqplot from arima residuals but im a bit lost about the underlying distribution, concretely I don't now how to interpret the tails.
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If there are covariant variables in the exponential distribution, how do I draw the QQ plot?

Observation Data X=x[1],... x[n]; covariant variables are eta=eta[1],...,eta[n]. The parameters of exponential distribution are composed of covariant variables(eta) and lambda. So, X[i]<-rexp(eta[i]...
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Confused about the statistical tests to choose or any transformation to apply

I am new to the stackexchange, so please forgive me for my editing ignorance. I am confused and stuck about how to proceed further with my data. I have the following data ...
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19 views

Is Linear regression good to such Q-Q plots

I am beginner to Machine Learning. So, I was looking at the Q-Q plot of underlying data on which i was thinking to apply linear regression. Could you please suggest below is my Q-Q plot. Whether is it ...
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460 views

Ways of visually comparing variance of data [closed]

I have a dataset containing 8 categories, I now want to visually compare the variance between the categories. The most common approach would be to use box plots or violin plots but I would like to ...
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57 views

An explanation of the choice of QQ-line in a normal QQ-plot

The standard precedure for making a QQ-line in a normal quantile-plot is to draw a line through the first and third sample quantile. My question is why? If data follows a normal distribution, then ...
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27 views

Approximating location and scale from QQ-plot

So I'm solving old exams in preparation for my statistical data analysis exam and came by this question. "b) Using the QQ-plot of the location-scale family that you have selected under part a, ...
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49 views

Testing normality with Confidence Intervals

I've read an article that talks about transforming non-normal data to normal data. In the article it says: "In this case, normality clearly cannot be assumed; the p-value is less than 0.05 and more ...
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Why don't my GWAS QQPlots look the same?

I am trying to create a QQPlot of 100 log-transformed p values from a GWAS study. The idea is that taking the -log(p) will magnify the smallest p values to make them easier to see. (reference) I was ...
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33 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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Reference line for QQPlots: robust regression vs quartiles method

Using the car package, I drew two qqPlots of 144 p values from correlated tests on one dataset. The top plot uses a reference line drawn by "quartiles" and the bottom plot uses a line drawn by "robust"...
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342 views

Interpreting Regression Diagnostic Plots

I'm sorry if this is a broad question but can someone explain to me how to interpret these regression diagnostic plots? I understand the Normal Q-Q show's how normal the spread of the data is, but the ...
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1answer
66 views

Introducing random slopes in nested model improves model fit but residuals variances become unequal

I have measured boldness scores (continuous variable) across time (trials) for individuals (ID) within colonies (colony). The data is coded such that individuals 1-30 belong to one colony, 31-60 to ...
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1answer
254 views

Interpreting a “wavy” QQ plot?

I am observing the following QQ plot produced from an OLS linear regression fit of my data: Many other SE questions discussion QQ plot interpretation, but this is an extremely regular (but non-...
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114 views

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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226 views

Strange qqplot for poisson gam

I've built a gam using mgcv and the following code ...
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92 views

How long is a distribution considered normal?

I have a dataset of metric distances ($n=5800$) and plotted those as a histogram. My initial thought was that this distribution looks normal. But after performing a Shapiro Wilk test and plotting the ...
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68 views

Non-parametric tests for interaction effects

I have two datasets that are both organised as follows: an algorithm calculated a quality score (Q) of a specific image (image) that was processed using a certain algorithm (method). One has 315 ...
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133 views

Assuming heteroscedasticity produces terrible QQ plot

I created a model using the lm function and obtained a model with an AIC of 390516.1 and the following QQ-plot: However, the Breusch-Pagan Test returned a p-value ...
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69 views

Granularity in a Normal Quantile Plot

I'm having some trouble understanding the concept of granularity so I'd like to have some clarity in this regard. After producing a normal Q-Q plot of some data (seen below), a question has asked me ...
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163 views

Q-Q plot, is this a approx normal distribution?

I searched online and looked video tutorials but I'm still not sure. Would you consider the below data normally distributed? I know the ideal fit in theory would be that most of the points are on the ...
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421 views

Need help interpreting a chi-square Q-Q plot for multivariate normality

I would like to know if my interpretation is correct with respect to the Q-Q plot I made (see the first image below). The second image is a plot of a chi-square distribution. At the first half, it ...
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82 views

Remove points from QQ plot of genome-wide association study

I have results from a genome-wide association study, which is basically a bunch of univariate tests (akin to linear regression). Results include statistical significance of each test, and thus I want ...
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1answer
51 views

What can we do given percentile values of the data? [closed]

We are given 20 percentile values say equally spaced percentiles 0 to 100 and also the sample size. Basic things we could do is figure out if this is normal using qq plot, other than that what else ...
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120 views

Is this QQ convex or concave?

With my data I got this exponential QQplot : is this QQplot convex or concave ? Thanks in advance
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156 views

A question about qqplot

This is my qq plot : Its concave-convex curve so it indicates light tails. But my mean excess plot : is increases which means the tail of the distribution of my data is heavy-tailed. I don't ...
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1k views

How to create a QQ plot compared to a function I define?

I want to create a QQ plot with my sample data compared to the PDF $e^{-y}$. My $y > 0$. My first approach was to create a random vector of values > 0, then plug those into $e^{-y}$ to get the ...
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QQ plot and $x = y$ line

If my qqplot is linear but the gradient is not the same as the 45 degrees line, what does this suggest? I am trying to examine the fit of laplace distribution to my sample data, so I randomly ...
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309 views

Fat Tails and Volatility clustering relationship

Let's say I have a return time series that after a proper ARMA modelling exhibits fat-tallness from QQ-plot. Can this be a consequence of volatility clustering so that by applying a GARCH model I ...
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34 views

Q-Qplot with outliers on the same side?

What does it mean when there are outliers in a Q-Q plot on the same side of the plot at both ends of the tails? I've attached the example I have, which is showing the residuals for the abundance of my ...
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121 views

Mismatch between KS test and qq plot?

I read many posts about the Kolmogorov-Smirnov test, Q-Q plots and CDFs. I decided to ask my own question about some difficulties I have with some real data. The data I'm using is available on this ...
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207 views

What is the algorithm for computing quantiles? [duplicate]

According to the Matlab documentation for qqplot, the ith ordered value is plotted against the ...
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53 views

What is a qq plot? Is it possible to make a qq plot between age (ranging from 20 to 70) and salary (ranging from 5000 to 10000)?

What I have understood is that it is a graphical technique used to check if 2 data sources (even if not from the same population) have similar data distribution. In that case the scatter plot will be ...
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1answer
78 views

Correct approach to carry out Q-Q Normal plot in R?

I am attempting to test some sets of data for normality. I have 64 groups to test. Each group has n=8 samples. [[I am aware of the problems with low n in regard to normality testing]] My end goal is ...
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110 views

Advice on negatively skewed residuals

I am running a simple one-way ANOVA of a continuous dependent variable and my two-level categorical independent variable. n value = 408; 198 in my control condition and 210 in my experimental ...
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2answers
284 views

Transform variables so residuals become normally distributed

I'm doing a project which involves multiple regression, and I'm aware (as far as I understand) that the residuals should be normally distributed with a constant variance. From my study, the residuals ...
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36 views

Does taking Ln of dependent variable improve assumptions of a multiple regression?

I have run two multiple regressions in R, one with average abundance as dependent variable, and one with log avg abundance as dependent variable. With the given plots, can we say one of the two ...
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501 views

What are the 'critical' values of skewness and kurtosis for normality assumption? [duplicate]

I am analyzing buy-and-hold abnormal returns of stocks (dependent variable) using OLS regression. These returns, however, tend to be positively skewed (and are so in my case). The residuals obtained ...
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2answers
171 views

Groups of Points in QQ Plots

While using QQ Plots, I always notice these 'groups' of points; many sets of points arranged in a straight line and sometimes in a curve as shown below: exponential: normal: The theoretical ...
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1k views

Identifying the normality by QQ plot

I have three variables and I want to know which of them can be considered to have the normal distribution. Variable A Variable B Variable C
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How to interpret negative quantiles on a qqplot?

I am having troubles interpreting the qq plot below. The histogram is residuals from linear regression. How to interpret negative theoretical and sample quantiles? Shouldn't it be [0,1]?
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140 views

Making a qqplot in R for functions

The question is: given that I have a data set and a theoretical cumulative distribution defined as a function, how can I make a qqplot to compare them? I cooked up a solution as follows: I chose a ...
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196 views

Is the assumption of constant variance violated in the following?

Is the assumption of constant variance violated in the following? I am measuring point in a 3D coordinate system (X,Y,Z) using 10 different methods. One method is the golden standard (reference). The ...