The best replacement for main effect is interaction.

The best alternatives to main effect are rated as following:
★★★★★  interaction
★★★  interaction effect
★★  simple main effect
★  simple effect
  effect

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Speaking about main effect, one probably shouldn't forget about an interaction.

Other search terms related to main effect and their ratings are:
★★★★★ an interaction
★★★ interaction
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main effect
interaction
interaction effect