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Effect sizes
How astrology compares with other approaches

Abstract -- An introduction to effect sizes in astrology and how they compare with those in other approaches such as graphology, palmistry, and psychological tests. Here effect sizes are expressed as a correlatiion, which ranges from +1 (perfect correlation as in feet vs metres) through 0 (no correlation as in two coin tosses) to -1 (perfect inverse correlation as in nightlength vs daylength). The results are also shown in terms of the equivalent hit rates, which for readers unfamiliar with correlations may be easier to understand. Astrology is said to fit reality so well that its effect size must be close to +1, equivalent to a hit rate close to 100%. But examples show how the fit is due to two non-astrological factors, namely (1) the ambiguity of astrological symbolism, so any chart can be made to fit any person after the event, and (2) hidden persuaders such as generalities-that-we-see-as-meaningful (the Barnum effect), so any person can see something that fits them in any reading. These factors make astrology seem to work even when the wrong chart is used. When these factors are controlled, for example by asking astrologers to match charts to owners whose identity is concealed, the observed effect sizes are not usefully different from zero. Accepted approaches such as personality and aptitude tests are better, usually much better. Especially telling is the negligible agreement between astrologers, because astrology seems pointless if astrologers cannot usefully agree on what a given birth chart indicates. Nevertheless astrology offers images of undeniable appeal that can feed the inner person in the same way that music, poetry and art do. There is more to astrology than having a useful effect size.