Set global contrasts
These functions are simple wrappers to set contrasts globally via options(contrasts = ...).
set_sum_contrasts() set_deviation_contrasts() set_effects_contrasts() set_default_contrasts() set_treatment_contrasts()
set_deviation_contrasts and set_effects_contrasts are wrappers for set_sum_contrasts. Likewise, set_default_contrasts is a wrapper to set_treatment_contrasts().
nothing. These functions are called for their side effects to change the global options.
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