Median Absolute Deviation vs Standard Deviation - Cross Validated most recent 30 from stats.stackexchange.com 2019-06-26T08:20:49Z https://stats.stackexchange.com/feeds/question/149745 http://www.creativecommons.org/licenses/by-sa/3.0/rdf https://stats.stackexchange.com/q/149745 7 Median Absolute Deviation vs Standard Deviation David https://stats.stackexchange.com/users/46830 2015-05-04T22:14:00Z 2019-02-13T11:53:53Z <p>To measure spread we use Variance or Standard Deviation. Variance and hence Standard deviation uses <strong>mean</strong> to find out the spread.</p> <p>Recently came across the MAD (median absolute deviation). <a href="http://en.wikipedia.org/wiki/Median_absolute_deviation" rel="noreferrer">http://en.wikipedia.org/wiki/Median_absolute_deviation</a></p> <p>Why is Median absolute deviation not as popular as Standard Deviation, although it looks more robust (immune to outlier)? In other words why are artifacts which measure the SD from the median are not as popular as the artifacts which measure it from Mean?</p> https://stats.stackexchange.com/questions/149745/-/329378#329378 4 Answer by Ben for Median Absolute Deviation vs Standard Deviation Ben https://stats.stackexchange.com/users/173082 2018-02-19T05:27:30Z 2019-02-13T11:53:53Z <p><strong>Robustness to outliers is a double-edged sword:</strong> Sometimes we want to estimate things in a way that is robust to outliers, which means that we do not mind getting large outliers. At other times we want to avoid large outliers, so we want to estimate things in a way that is <em>not</em> robust to outliers. Similarly, with measures of spread, sometimes we want something that is robust to outliers, so that large outliers do not increase the measure. At other times we want our measure of spread to reflect the presence of large outliers by manifesting in a larger value.</p> <p>In decision-theory, issues like this are dealt with by specifying a <strong>penalty/loss function</strong> which penalises you for your error in estimation of a quantity. Two common loss functions are absolute-error loss and squared-error loss (shown in the following plots, taken from <a href="https://stats.stackexchange.com/questions/147001/147044#147044">this answer</a> by <a href="https://stats.stackexchange.com/users/42380/jean-paul">Jean-Paul</a>).</p> <p><img src="https://i.stack.imgur.com/PtfUm.png" alt="Absolute-error and squared-error penalty functions"></p> <p><strong>Absolute-error loss</strong> penalises you according to the <em>absolute</em> deviation of your estimate from the true value. This form of loss function leads to estimation using medians. This form of loss function is robust to outliers in the sense that outliers contribute a penalty that is proportionate to their size. Measures of spread in this context reflect the expected loss of a particular estimate of central location, with the expected loss being a weighted sum of absolute deviations from the estimated central location.</p> <p><strong>Squared-error loss</strong> penalises you according to the <em>squared</em> deviation of your estimate from the true value. This form of loss function leads to estimation using means. This form of loss function is sensitive to outliers in the sense that outliers contribute a penalty that is proportionate to their squared deviation - this magnifies the effect of large outliers. Measures of spread in this context reflect the expected loss of a particular estimate of central location, with the expected loss being a weighted sum of squared deviations from the estimated central location.</p> <hr> <p>In regard to the choice between <strong>median absolute deviation</strong> and <strong>standard deviation</strong> these same considerations apply. The former measure is a measure of spread that represents expected absolute-error loss, and is more robust to outliers. In this case, outliers do not manifest in large increases in the measure of spread. The latter is a measure of spread that represents expected squared-error loss, and is more sensitive to outliers. In this case, the outliers will manifest in large increases in the measure of spread.</p>