Rasch simultaneous vertical equating for measuring reading growth.

The longitudinal measurement of ability growth requires that the measures that are taken at the various time points be obtained using the same yardstick. Tests given for this purpose, therefore, need to be equated. The popular practice still in use for the purpose of equating tests, is the grade-equivalent. This paper compares the observation of children's growth in reading using grade-equivalents with that using Rasch Simultaneous Vertical Equating procedure. It is found that grade equivalents differ much more between two different test forms compared to ability measures obtained using Rasch Simultaneous Vertical Equating. It is also found that the spread of students' grade-equivalents, increased over the years as they grow while the standard deviations of their Rasch measures remain relatively constant over the same period of time. Student responses to the ITBS Form 7 and the CPS90 and CPS 91 were used. A total of 5,623 students were tracked over eight years.