Units of change in a developmental variable during a specified time interval. It is obtained by subtracting two age-related values from each other. For example, if a group of infants had a mean score of say 10 seconds for looking at a display followed by 15 seconds at the next age, then the measure of mean velocity between these two ages (or the unit of change) would be +5 seconds. It can also be the case that the velocity value obtained is negative. Such a measure of developmental velocity can sometimes be more informative than just the group mean in certain studies (e.g., in those comparing the development of fullterm and preterm infants), showing that the speed or tempo of development is faster for one group compared to another.
See Developmental acceleration, Developmental function, Developmental trajectories