The second criterion is not met for this case. Grubbs’ outlier test produced a p-value of 0.000. Clearly, outliers with considerable leavarage can indicate a problem with the measurement or the data recording, communication or whatever. the decimal point is misplaced; or you have failed to declare some values I have 400 observations and 5 explanatory variables. If you use Grubbs’ test and find an outlier, don’t remove that outlier and perform the analysis again. If I calculate Z score then around 30 rows come out having outliers whereas 60 outlier rows with IQR. Determine the effect of outliers on a case-by-case basis. The issue of removing outliers is that some may feel it is just a way for the researcher to manipulate the results to make sure the data suggests what their hypothesis stated. Outliers, Page 5 o The second criterion is a bit subjective, but the last data point is consistent with its neighbors (the data are smooth and follow a recognizable pattern). o Since both criteria are not met, we say that the last data point is not an outlier , and we cannot justify removing it. Data outliers can spoil and mislead the training process resulting in longer training times, less accurate models and ultimately poorer results. I'm very conservative about removing outliers, but the times I've done it, it's been either: * A suspicious measurement that I didn't think was real data. If new outliers emerge, and you want to reduce the influence of the outliers, you choose one the four options again. The output indicates it is the high value we found before. Sometimes new outliers emerge because they were masked by the old outliers and/or the data is now different after removing the old outlier so existing extreme data points may now qualify as outliers. For example, a value of "99" for the age of a high school student. Then decide whether you want to remove, change, or keep outlier values. You should be worried about outliers because (a) extreme values of observed variables can distort estimates of regression coefficients, (b) they may reflect coding errors in the data, e.g. Because it is less than our significance level, we can conclude that our dataset contains an outlier. Dataset is a likert 5 scale data with around 30 features and 800 samples and I am trying to cluster the data in groups. Another way, perhaps better in the long run, is to export your post-test data and visualize it by various means. $\begingroup$ Despite the focus on R, I think there is a meaningful statistical question here, since various criteria have been proposed to identify "influential" observations using Cook's distance--and some of them differ greatly from each other. 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