Statistical Analysis Stack Exchange
Statistical Analysis Stack Exchange

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 interpret a QQ plot

I am working with a small dataset (21 observations) and have the following normal QQ plot in R: Seeing that the plot does not support normality, what could I infer about the underlying distribution? It seems to me that a distribution more skewed…
JohnK
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Interpreting QQplot - Is there any rule of thumb to decide for non-normality?

I have read enough threads on QQplots here to understand that a QQplot can be more informative than other normality tests. However, I am inexperienced with interpreting QQplots. I googled a lot; I found a lot of graphs of non-normal QQplots, but no…
greymatter0
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R - QQPlot: how to see whether data are normally distributed

I have plotted this after I did a Shapiro-Wilk normality test. The test showed that it is likely that the population is normally distributed. However, how to see this "behaviour" on this plot? UPDATE A simple histogram of the data: UPDATE The…
Le Max
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PP-plots vs. QQ-plots

What is the difference between probability plots, PP-plots and QQ-plots when trying to analyse a fitted distribution to data?
kay
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Interpretation of plot (glm.model)

Can anyone tell me how to interpret the 'residuals vs fitted', 'normal q-q', 'scale-location', and 'residuals vs leverage' plots? I am fitting a binomial GLM, saving it and then plotting it.
Summer
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What is the use of the line produced by qqline() in R?

The qqnorm() R function produces a normal QQ-plot and qqline() adds a line which passes through the first and third quartiles. What is the origin of this line? Is it helpful to check normality? This is not the classical line (the diagonal $y=x$…
Stéphane Laurent
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Benefits of using QQ-plots over histograms

In this comment, Nick Cox wrote: Binning into classes is an ancient method. While histograms can be useful, modern statistical software makes it easy as well as advisable to fit distributions to the raw data. Binning just throws away detail that is…
MvG
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How to tell if my data distribution is symmetric?

I know that if the median and mean are approximately equal then this means there is a symmetric distribution but in this particular case I'm not certain. The mean and median are quite close (only 0.487m/gall difference) which would lead me to say…
user72943
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How to interpret a QQ-plot of p-values

I am doing GWAS SNP association studies on diseases by using a software called plink (http://pngu.mgh.harvard.edu/~purcell/plink/download.shtml). With association results I get p-values for all the SNPs that was analyzed. Now, I use a QQ-plot of…
eXpander
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Which to believe: Kolmogorov-Smirnov test or Q-Q plot?

I'm trying to determine if my dataset of continuous data follows a gamma distribution with parameters shape $=$ 1.7 and rate $=$ 0.000063. The problem is when I use R to create a Q-Q plot of my dataset $x$ against the theoretical distribution gamma…
user22119
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What is the basis for the Box and Whisker Plot definition of an outlier?

The standard definition of an outlier for a Box and Whisker plot is points outside of the range $\left\{Q1-1.5IQR,Q3+1.5IQR\right\}$, where $IQR= Q3-Q1$ and $Q1$ is the first quartile and $Q3$ is the third quartile of the data. What is the basis for…
Tavrock
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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 shows clearly that the tails are not fitted…
Stat Tistician
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Removing extraneous points near the centre of a QQ-plot

I'm trying to plot a QQ-plot with two data sets of about 1.2 million points, in R (using qqplot, and feeding the data into ggplot2). The calculation is easy enough, but the resulting graph is painfully slow to load, because there's so many points.…
naught101
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My distribution is normal; Kolmogorov-Smirnov test doesn't agree

I have a problem with the normality of some data I have: I've done a Kolmogorov test which says it isn't normal with p=.0000, I don't understand: the skewness of my distribution =-.497, and the kurtosis =-0,024 Here is the plot of my distribution…
Boo
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Confidence bands for QQ line

This question doesn't specifically pertain to R, but I chose to use R to illustrate it. Consider the code for producing confidence bands round a (normal)…
user603
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