Learn in the textbookRegression, Correlation, and Spurious Correlation
DefinitionIn correlation and causation (相関と因果), correlation means two quantities rise and fall together, while causation means one brings about the other, and a correlation does not by itself prove causation.
ExplanationA chart that puts two quantities on the horizontal and vertical axes and plots the data as dots is a scatter diagram (散布図); dots rising to the right show a positive correlation (正の相関), and dots falling to the right show a negative correlation (負の相関). The direction and strength are given by the correlation coefficient (相関係数), which ranges from -1 to +1, with values near 1 in absolute terms meaning a strong relationship and values near 0 a weak one. To claim causation, it is not enough that the two move together; you must confirm that changing one actually changes the other.
AnalogyTwo people who ride the same train every morning always show up at the station together, yet neither one is bringing the other.
Memory hookThe Japanese 相関 starts with a character meaning "mutual", while 因果 literally reads "cause and fruit", so the word itself separates the cause from the result.