The phrase "to measure the difference" means to compare two or more things to see how they are not the same or how much they vary from each other. Full definition
Between - group effect sizes were determined by measuring the difference in magnitude of change between treatment and comparison groups in units of standard deviation. (ebmh.bmj.com)
Already, the bond yield curve, which measures the difference between short - term interest rates and long one, has been rising. (fortune.com)
The project also noted a significant «enforcement gap,» which measures the difference between the laws on the books and how well they're actually implemented. (publicintegrity.org)