Statistics
Producing IQR and Outlier statistics with SQL
The interquartile range (IQR) measures the spread of the middle 50% of a distribution — the distance between the first quartile (Q1) and the third quartile (Q3). Combined with Tukey’s 1.
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Producing Mode statistics with SQL
The mode is the value or values that appear most frequently in a dataset. Unlike the mean or median, it applies naturally to categorical and ordinal data — star ratings, product codes, survey responses — and reveals what is most common, not what is average.
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Producing Alternative Means statistics with SQL
MySQL’s built-in AVG() computes the arithmetic mean — the sum divided by the count. That is the right default for many questions, but it is not always the right measure of central tendency.
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Producing One-Sample Z-Test statistics with SQL
The one-sample Z-test determines whether a sample mean differs significantly from a known population mean when the population standard deviation is also known. It is the appropriate test when the population parameters are established — quality control benchmarks, national averages, long-run process measurements — and you want to evaluate whether a new sample is consistent with them.
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Producing Skewness statistics with SQL
Skewness measures the asymmetry of a distribution. A perfectly symmetric distribution has a skewness of zero. A positive skew (right-skewed) means the tail extends to the right — a small number of high values pull the mean above the median.
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Producing Chi-Squared statistics with SQL
The Chi-Squared test is one of the most widely used statistical tests for categorical data. It comes in two flavors: the goodness-of-fit test asks whether an observed frequency distribution matches an expected one, while the test of independence asks whether two categorical variables are associated with each other.
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Producing Two Sample T-Test statistics with SQL
The two sample t-test for equal variance is a statistical test to determine if the means of two groups are different enough that the difference is likely caused by some underlying difference, rather than random chance.
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