This is an assumption of an ANOVA with a repeated measures factor, where violations of this assumption can invalidate the analysis conclusions.
Knowledge of the sphericity of the Earth survived into the medieval corpus of knowledge by direct transmission of the texts of Greek antiquity (Aristotle), and via authors such as Isidore of Seville and Beda Venerabilis.
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Learned Christian authors such as Basil of Caesarea, Ambrose and Augustine of Hippo were clearly aware of the sphericity of the Earth.