Why does FastBridge use the median score for WCPM and accuracy in CBMreading when used for screening?

When students complete the FastBridge assessment CBMreading as part of universal screening, they read three different passages. The primary outcome score for CBMreading is Words Read Correctly per Minute (WRCM). The student’s accuracy on each passage is also recorded and shown in certain reports. The middle of the three WCPM scores is shown in the FastBridge screening reports. The middle score is also known as the median score.  

Using the median score has a long and psychometrically-based reason. Every time we measure anything, there is some error in the measurement. To account for this error, multiple measures are used. One way of selecting which of multiple measures to use is to compute an average (mean). Unfortunately, means are not good indicators of the available data when there are few data points. Since CBMreading (and this is true for all publishers, not just FastBridge) collects scores from 3 stories, there are too few scores to calculate an average. One solution would be to have students complete 10 or 20 stories, but that would be a lot of work. Instead, CBM researchers showed that using the median (middle) score of 3 samples is the best way to estimate a student's current true skills. This method takes into account that students might have very bad or very good days and taking the middle score gives us the best idea of the performance that falls between the lowest and highest. Using the highest score would overestimate the student's true skills and using the lowest score would underestimate the student's skills.

The FastBridge system calculates each of the three WRCM scores by subtracting any errors from the total words read in 1 minute. The accuracy is calculated by dividing the WRCM by the total words read. When accuracy is displayed on a FastBridge report, it appears as a percentage. For CBMreading scores, the accuracy displayed will always be matched to the passage corresponding to the median score. This method promotes a valid interpretation such that, "When Timmy read 86 WRCM he read with 93% accuracy"

The following table shows data from three CBMreading passages including the total words read (TWW), errors, WRCM, and accuracy. The highlighted values are the median score and accuracy that would be displayed on FastBridge screening reports.