The best replacement for variation is variance.

The best alternatives to variation are rated as following:
★★★★★  variance
★★★★  standard deviation
★★★★  deviation
★★★  adaptation
★★★  variability
★★  variations
★★  inverse variation
★  proportion
★  dispersion
★  mutation
★  variant
  continuous variation
  discontinuous variation
  extension
  correlation coefficient
  equivalent variation
  covariance
  indirect variation
  complementary distribution
  declination
  constant variation
  heredity
  variation
  relative standard deviation
  linear equations
  non heritable variation

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Speaking about variation, one probably shouldn't forget about repetition.

Other search terms related to variation and their ratings are:
★★★★★ repetition
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variation
variance
standard deviation