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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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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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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
97 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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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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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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67 views

Strange qqplot for poisson gam

I've built a gam using mgcv and the following code ...
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87 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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33 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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53 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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50 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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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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113 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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51 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
37 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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62 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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90 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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1answer
514 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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952 views

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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Horizontal Q-Q norm Plot [duplicate]

I know that if the quantile plot between two random variables is a straight line, then the cumuluative distribution functions of the two are likely linear transforms. However, after trying to do a Q-...
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1answer
162 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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30 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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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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152 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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46 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
51 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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68 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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1answer
149 views

R: Transform variables so residuals become normally distributed

I'm currently stuck with a problem and would appriciate some help. I'm doing a project which involves multiple regression, and I'm aware of (as far as I understand) that the residuals should be ...
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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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1answer
337 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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190 views

How to match histograms to normal quantile plots?

Here is the problem I'm working on: I'm not sure if I'm right -- I have: Plot 1: D -- not sure why Plot 2: C (left skewed should have an upward quantile plot) Plot 3: A (because the bars are ...
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2answers
135 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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764 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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Which Q-Q plot model - residuals appears to be more normally distributed

Can you please explain to me if the following statement is true or false: Based on a comparison of the Normal Q-Q plots, Model 2 residuals appear to be a bit more normally distributed than the Model 1 ...
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797 views

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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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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164 views

Convention of QQ Plot Axes/ Axis Labels

I am building qq plots with a statistics package, however I notice my output is different than what I found in the econometric literature. For example, here is a qq plot from a publication I came ...
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2answers
100 views

Checking assumption of normality with multiple subsets of unequal size

I have a dataset consisting of 12 subsets, each subset contains between 10 and 15 temperature observations from one person. I would like to determine whether or not it is reasonable to assume ...
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139 views

Question regarding qq plot of transformed data; what's the right transformation to use

So I will post qq of original data first along with logarthmic transformed data. If I'm interpreting the first correctly it looks like there is right skew to the data. The log transformed has a 'bulge'...
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1answer
696 views

Why residual plots are used for diagnostic of glm

I asked this question here which @Glen_b had kindly answered. I thought I could get a more detailed explanation with references if I posted it as a whole new question. So my questions is why ...
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1answer
2k views

residual vs. QQ-plot in multiple regression

I'm working on a Kaggle multiple regression tutorial competition and inspecting plots of my residuals. I followed a suggestion and log transformed several independent variables and the dependent ...
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1answer
335 views

QQ-plot doesn't correspond with histogram

I made a histogram and QQ-plot, using this code: ...
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Mix Design: Failure to predict response

I have performed a mixture design containing 3 terms and I have carried out 6 experiments needed to adjust a quadratic model. It fitted the data quite well: ...
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179 views

Large Dataset: Is the data normally distributed?

Shapiro-Wilk and Anderson-Darling tests can be used to test normality on small data sets, however the data set I am using has over 100,000 points. Is there a test to check the distribution of a large ...
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1answer
1k views

Interpreting QQ plot of poisson regression

This is the QQ plot resulting after fitting a poisson regression. I found in a book saying that central line corresponds to zero cases in the response. I can imagine that for zero response cases ...
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55 views

QQ-Plot interpretation [duplicate]

I am working with a dataset (900 observations) and have the following normal QQ plot in R: Seeing that the QQ-plot little bit strange, what could I infer about the underlying distribution? It seems ...
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124 views

Does this normal distribution plot show a good fit? [duplicate]

I am not well acquainted enough to judge this well. This plot was obtained from normplot in Matlab. There is a good linear fit although the tails are concerning. Does my data follow a normal ...
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1answer
897 views

Shapiro-Wilk-Test with p < .05 but data looks normal

I'm working with R since a few months and read the book Discovering Statistics with R by from Andy Field until Chapter 12 by now. I have some data, which I (for now without any specific reason) want ...
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101 views

Overlapping data and QQ plot

Problem Have a time series of data and want to look at changes over long periods so want to use overlapping changes, then want to compare these overlapping changes to overlapping changes from a normal ...
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130 views

Is Probability Plot a member of QQ plot?

Even though I read my lecture notes and PP-plots vs. QQ-plots carefully, I'm still confused. So Probability plot is a plot of samples vs theoretical quantiles. There we can find a or some parameters ...