I think this is an excellent quote about assessing by Heward who is quoting Kauffman about the necessity to use data to make instructional decisions:
The teacher who cannot or will not pinpoint and
measure the relevant behaviors of the students he
or she is teaching is probably not going to be very
effective. . . . Not to define precisely and to measure
these behavioral excesses and deficiencies,
then, is a fundamental error; it is akin to the malpractice
of a nurse who decides not to measure vital
signs (heart rate, respiration rate, temperature, and
blood pressure), perhaps arguing that he or she is
too busy, that subjective estimates of vital signs are
quite adequate, that vital signs are only superficial
estimates of the patient’s health, or that vital signs
do not signify the nature of the underlying pathology.
The teaching profession is dedicated to the task
of changing behavior—changing behavior demonstrably
for the better. What can one say, then, of
educational practice that does not include precise
definition and reliable measurement of the behavioral
change induced by the teacher’s methodology?
It is indefensible. (p. 514)”
Heward, W. (2003).
THE JOURNAL OF SPECIAL EDUCATION VOL. 36/NO. 4/2003/PP. 186–205