Set the four labels in order and the classifying principle becomes visible, because they form an ascending series.

Intrapersonal communication involves one person only — thinking, self-talk, internal reasoning; sender and receiver are the same individual.

Interpersonal communication involves two people in face-to-face exchange, with immediate feedback flowing both ways.

Group communication involves a small, countable set of people — a committee, a seminar, a classroom — where each participant can still address the others directly.

Mass communication addresses a large, scattered and heterogeneous audience through a medium, with feedback that is delayed and indirect.

Moving along that list, the count of people involved rises from one to two to a few to many. The criterion generating the four types is therefore the number of participants in communication.

Medium used in communication yields a different classification altogether — verbal against non-verbal, or oral against written, or print against electronic — and none of those divisions produces the four names given.

Nature of the message produces categories such as formal and informal, or persuasive and informative.

Effectiveness is a matter of degree rather than of kind, so it grades communication rather than sorting it into types.

Hence the criterion is the number of participants in communication.