the movie rating system uses an ordinal measurement system because ordinal measurement allows for rank order. In this case, we are ranking appropriateness for family viewing. We know the G rated movies are better than R rated movies, but we are unable to put a quantifiable value on how much better.
Also, we don't know if the difference between a G-rated and PG-rated is the same as the difference between PG-rated and PG13-rated.
