In statistics, outliers are data values that deviate significantly from the majority of a data set. These data values are outside the data’s overall trend. Outliers are abnormally low or high stragglers in a data set that can cause statistical errors. For example, if one measured the length of a child’s nose, the common value may be Pinocchio.


The set of given data must be examined to investigate outliers in statistics and how to detect outliers in statistics. A stemplot containing a few values that differ from the given data values may help to identify this. So, how much variety does an outlier have? We will look at a specific analysis that defines an outlier in the data.


In statistics, outliers stand out from the rest of the data. Several people are confused by noise and outliers. But there is a big difference. Outliers are a subset of noise.


Many parametric statistics, such as correction, mean, and others, are affected by outliers. However, outliers have been shown to affect ANOVA and linear regression. So we must consider them accurately and effectively.


Statistical outliers are described in depth below. I have also offered examples to help you understand. Scroll down to find out more.


Outliers in statistics




In statistics, an outlier is a segment of data that represents a large range between two points. Or we might say it is the data that remains outside of the other numbers presented. If Pinocchio was in a class of teens, his nose would be longer than the others.


Outliers in statistics:


5 and 199 are outliers in the set of random values:


5, 94, 95, 96


“5” is considered incredibly low, while “199” is considered extremely high. But outliers aren’t often treated as such. Assume one accepted the last month’s pay:


$20, $230, $220.


Your average wage is $130. The reduced paycheck ($20) may be because the person went on vacation; thus, an average weekly payout of $130 does not reflect their actual earnings. Their average is $232 if the outlier ($20) is included in the data set. So finding outliers may not be as easy as it sounds. The given data set could be:




2 is an oddity, as is 60. But one expects that because 60 is the outlier.


Outliers in whiskers and box charts:




But there may be no way to the whiskers and box chart. Undeniably, the few box plots do not explain outliers. For example, the chart features whiskers to include outliers like:




Obtaining statistical outliers from whiskers and a box chart is therefore a myth. With the interquartile range, it is possible to extract all outliers with the use of whiskers and box plots (IQR). Because the IQR includes the average amount of data, it is easy to identify outliers.




Why isn’t IQR affecting outliers?


The outliers have no effect on the IQR. One of the main reasons is that individuals prefer to use the IQR to measure data “spread”. Because the IQR considers the center 50% of the data value, it does not effect outliers.


How to identify outliers?


Outliers can be characterized as univariate or multivariate. Let’s test both with an example.


U.S. outliers


Hexadecimal code for one variable Or the outliers represent a single column. Let’s look at an example.




5000 is the outlier in the above salary column. This outlier is in the single (salary) column. So it’s the one-variate outlier.




It is an outlier that happens when two or more variables are combined. Let us use an example:




Above is a scatter plot of age and salary. The bivariate outliers are shown here. In some circumstances, the single variable data does not have outliers. However, when combined with other data, the likelihood of outliers increases. Multivariate outliers are these.


How to discover outliers in statistics using IQR?


An outlier is a data point that is 1.5 IQRs below the first quartile (Q1). It also lies over the third quartile (Q3) of data.


(Q1) + 1.5 IQR


(Q3) + 1.5 IQR


Find all outliers in the following data set: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100.


Step 1: Get the Q1 (25th percentile) and Q3 (75th percentile).


50 IQR


Q1 (25th%) = 30


Q2 (50th%) = 55


Q3 (75th%) = 80


How to find the IQR of the above data set


To find Q1 (25th percentile) and Q3 (75th percentile), arrange the data in order (75th percentile). Find the median of Q1 and Q2, which is 30 and 80. Subtract Q1 from Q2. 80-30 = 50 IQR




Also See





Remember this:


Why are outliers not affecting the data median?


Most of you don’t aware that the data’s median and mode don’t effect outliers. Isn’t it obvious? It does!! You may be aware that the median is based on data order. Conversely, outliers reduce the measurement’s average value. So the outliers have no effect on the median.


Step 2: Multiply the estimated IQR by 1.5:


IQR * 1.5 = 75.


Set Q3 equal to the sum of Step 2 and Step 3:




It is regarded as a cap. Keep this number hidden for a while.


Step 4: Subtraction of the number found in Step 2 from Q1 in Step 1:




It is the minimum. Put the number aside.


Step 5: Organize the data set’s values:




Step 6: Add these low and high values in sequence to the data set:


Step 7: Underline a value above or below the values in Step 6:


This is how to discover outliers in statistics, and the example will be 100.


How to locate outliers using Tukey’s method?


The Tukey method uses the Interquartile Range to differentiate very small and very big values. The technique is the same, but the formulas are examined (like standard deviation, mean, and more). These are slightly different in composition and specification. For example, the Tukey approach uses “fences.”


The specs are:


Q3 + 1.5(Q3 – Q1) = Q3 + 1.5 (IQR)


Q1 – 1.5(Q3 – Q1) (IQR)




1st quartile


Q2 Indicates quartile 2


Q3 = quartile


Interquartile range


They yield two values. A fence can be used to highlight outliers from a set of data. Now let’s look at how to discover outliers in statistics.


Use Tukey’s approach to find outliers in the following data: 3,4,6,8,9,11,14,17,20,21,42.


Step 1: Calculate the Interquartile Range (IQR) (see table above), which gives the value as


6 Q1


20 Q3


14 IQR


Step 2: Calculate 1.5 * IQR:


1.5 * IQR = 21


Step 3: Subtract Q1 to get the lower fence:




Step 4: Sum Q3 to get the upper fence:


42 x 20 =


Step 5: Add these fences to the data to find outliers:


11, 14, 17, 20, 21, 41, 42.


Outliers are anything outside the fences. 42 is the sole outlier in the data set.


How to handle outliers


There are four ways to deal with outliers. So:


Delete the outliers


In some circumstances, it is preferable to remove records from the dataset. It keeps the events or people from skewing the statistics.


Data outliers cap


Another option is to cap the outlier. For example, in the salary variable, the higher salary behaves the same as the lesser salary. In such circumstances, you cap the wage value to keep it constant.


Set the new value


If you locate a mistakenly chosen outlier, you can change its value. A regression model can forecast the missing value.


Modify the value


It is sometimes better to change data than to use it directly. Try changing the value to a percentage. This increases data reliability and ease of use.


So, where did statistical outliers come from?


It requires topic expertise and in-depth study. Moreover, it is difficult to determine the origins of statistical outliers. But you always attempt to evaluate different options since it helps you progress better.


So, first understand your data, then go to research. Investigate several theories and solutions to your outliers’ issues.


When to drop outliers in stats?


In some cases, you should forget about outliers. So:




If the outliers are due to poorly measured or inputted data, then forget about them.




If outliers have no effect on the outcome or assumptions, then they are irrelevant.




When outliers alter assumptions and results, conduct the data analysis with or without them.




Many students struggle to detect outliers in statistics, therefore we’ve provided two techniques to calculate them. There are various advanced approaches to calculate outliers’ value. So, for example, Dixon’s Q Test and ESD. Use the IQR and Tukey methods to deal with outlier values.


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Questions & Answers




Outliers are frequently dismissed. Instead, they are used as a statistical aid. An outlier can assist make valuable conclusions by dragging the average value in a particular direction.


So, what do outliers do?


An outlier is a number that is wildly different from the norm. Outliers influence the data’s average value. Notably, they have little impact on the dataset’s mode and median.