Simpson's paradox
A relationship that points one way within every group and the other way when the groups are pooled.
A fourth trap catches experienced analysts. Within every group, a relationship can point one way while the pooled data points the other: Simpson's paradox. Suppose that in both of the shop's warehouses, faster packing comes with fewer complaints. But the busy city warehouse packs fast and gets many complaints from its sheer volume of awkward orders, while the small rural one packs slowly and gets few. Pool the two and "faster packing" seems to go with "more complaints".
Try it now
A straight-line fit with R², which for one variable is the correlation squared. Drag a single point far from the others.
Learn it properly
Correlation and the shape of data · Covariance and correlation, curves, outliers and common causes, rank correlation, skew and kurtosis. About 30 minutes.
Other terms from the same lesson
- correlation: Covariance divided by both standard deviations: a number from -1 to 1 for how closely two variables follow a straight line.
- kurtosis: How heavy a distribution's tails are: how often extreme values appear compared with a normal distribution.
- skewness: How lopsided a distribution is. Right-skewed data has a long upper tail, which pulls the mean above the median.
- Spearman correlation: Pearson's correlation computed on ranks: it measures whether two numbers always move the same way, and resists outliers.
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