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It can get rid of skewness Before log-transformation After log-transformation 0 2 4 6 8 10 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35-2.5 0.0 2.5 2. Therefore, right skewness is positive skewness which means skewness > 0. If it’s unimodal (has just one peak), like most data sets, the next thing you notice is whether it’s symmetric or skewed to one side. In a well-known article, Delong and Summers (1985) studied whether business cycles are symmetrical by applying the skewness coefﬁcient to GDP, industrial production, and the unemployment rate. Joint test for Normality on e: chi2(2) = 18.29 Prob > chi2 = 0.0001 Joint test for Normality on u: chi2(2) = 1.36 Prob > chi2 = 0.5055 model 2 Tests for skewness and kurtosis Number of obs = 370 Replications = 50 (Replications based on 37 clusters in CUID) The first thing you usually notice about a distribution’s shape is whether it has one mode (peak) or more than one. Stanislav Kolenikov, 2006. Skewness – Skewness measures the degree and direction of asymmetry. Skewness. Figure 2: Returns are stored in a column. Alternative Hypothesis: The dataset has a skewness and kurtosis that does not match a normal distribution. However, because the sampling distribu-tion of the skewness coefﬁcient for serially correlated data is of 0.0010 for skewness, 0.0442 for kurtosis, and 0.0024 for the joint univariate test. Figure 1: Returns are stored in a row. Determining if skewness and kurtosis are significantly non-normal. Taking logs of the data is often useful for data that are >0 because: 1. Stata does not provide a command to calculate the skewness in this situation. gra res, normal bin(50) /* normal option superimposes a normal distribution on the graph */ Residuals show signs of right skewness (residuals bunched to left – not symmetric) and kurtosis (leptokurtic – since peak of distribution higher than expected for a normal distribution) The univariate tests of the other three variables do not lead to a rejection of the null hypothesis of normality. The following Stata commands will do the job. Their histogram is shown below. "MVSKTEST: Stata module to test for multivariate skewness and kurtosis," Statistical Software Components S456701, Boston College Department of Economics. The scores are strongly positively skewed. Negative (Left) Skewness Example. This first example has skewness = 2.0 as indicated in the right top corner of the graph. Skewness. A symmetric distribution such as a normal distribution has a skewness of 0, and a distribution that is skewed to the left, e.g., when the mean is less than the median, has a negative skewness. If assumptions of t-test violated, transform data so that t-test can be applied to transformed data. The following code shows how to perform this test: jarque.test(data) Jarque-Bera Normality Test data: data JB = 5.7097, p-value = 0.05756 alternative hypothesis: greater The p-value of the test turns out to be 0.05756. Others use simple statistics to test skew-ness. Another variable -the scores on test 2- turn out to have skewness = -1.0. The question arises in statistical analysis of deciding how skewed a distribution can be before it is considered a problem. Check histogram of residuals using the following stata command . If returns are stored in a row. 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