Parts per thousand (ppt), also known as the “relative standard deviation”, is useful when comparing the uncertainty between different measurements of varying magnitude (i.e. it is a measure of the precision within an experiment.) Parts per thousand can be applied to any set of data where more than one experimental value has been applied – i.e. volumes, percentages, concentrations, etc.We will use parts per thousand often this year, so knowledge of how it works is critical for the successful student.