On the 17 item HAM-D here are the typical score ranges [1]
Not depressed: 0–7
Mild (subthreshold): 8–13
Moderate (mild): 14–18
Severe (moderate): 19–22
Very severe (severe): >23
So, a 4 point difference vs placebo is what I would consider a "clinically significant" improvment, e.g. about the difference between mild vs. moderate, or moderate to severe.
Not depressed: 0–7 Mild (subthreshold): 8–13 Moderate (mild): 14–18 Severe (moderate): 19–22 Very severe (severe): >23
So, a 4 point difference vs placebo is what I would consider a "clinically significant" improvment, e.g. about the difference between mild vs. moderate, or moderate to severe.
https://en.wikipedia.org/wiki/Hamilton_Rating_Scale_for_Depr...