documentation_cmp
Prefix metadata fields with == or != to limit comparisons to within or
between groups, respectively. For example, stat.by = '==Sex' will
run calculations only for intra-group comparisons, returning "Male" and
"Female", but NOT "Female vs Male". Similarly, setting
stat.by = '!=Body Site' will only show the inter-group comparisons, such
as "Saliva vs Stool", "Anterior nares vs Buccal mucosa", and so on.
The same effect can be achieved by using the within and between
parameters. stat.by = '==Sex' is equivalent to
stat.by = 'Sex', within = 'Sex'.