Baseline
A stable measure of a behaviour before any intervention is introduced.
A baseline is the data you collect before you do anything. It tells you where the learner actually is, not where you assume they are. Without it, you have no way of knowing whether your intervention is working, making things worse, or irrelevant.
A usable baseline needs to be stable - that means the data aren't still climbing or falling sharply when you start your intervention. If you intervene during an upward trend, any improvement could be the natural trajectory, not your teaching. If the data are still settling, you're measuring noise.
The ethical tension with baselines is real. Waiting for stability means delaying support for someone who is struggling. The answer isn't to skip the baseline - it's to use your measurement system to detect stability faster and to understand what you're actually intervening on.