Comparison. These judgements are based upon reseach assessing thousands of kids patterns and generating statistical analysis. Then standard milestones are represented. Your pediatrician then compares your child's progresses to these patterns.
By skills shown. The peabody motor developmental scales, usually administered by occupational and physical therapists, assess both fine (arms/ shoulder girdle/upper torso) and gross ( lower torso/ legs) abilities related to norms for particular ages. It is important to observe both the quality and quantity of movements. There are other ways your pediatricians can check for problems, so report your concerns .