Hello Michael,
Thank you for your question. As you may be aware, CODAP descends from Fathom both in spirit and, in some places, in code. CODAP computes Q1 and Q3 as Fathom does. The decision for both applications is based on the tenet that you should get the same value for each as you get from the function that computes the 25th or 75th percentile. The percentile function takes any number between 0 and 1 for an argument, does not treat 0.25 or 0.75 as special, and always includes all values.
Perhaps the fact that there are multiple multiple methods for determining the quartile values has at least the saving grace of providing an opportunity for students to experience the arbitrariness of some rules and to grapple with a situation in which, at least some would say, there is more than one right answer.
Further thoughts welcome!
Bill