let $a$ be the number of treatments.
In Randomized Complete Block Design(RCBD), degrees of freedom due to treatment is $(a-1) $ .
let $k$ be the number of treatments exactly in each block in a Balanced Incomplete Block Design(BIBD).
In BIBD, Adjusted Mean Square due to treatment is given by adjusted sum of square due to treatment divide by degrees of freedom $(a-1) $.
But my question is why do we divide it by $ (a-1) $? .Since it is adjusted , why don’t we divide it by $(k-1) $?