If the Project Manager gives an instruction to reduce the quantity of works, is it a compensation event? If yes, how can we assess the reduced price using define cost when we can remeasure the amount of works done by the Contractor using the BOQ rate.
It is a compensation under 60.1 (1) because the PM has given an instruction and this is one of the express exclusion for it to be a compensation event under 60.4.
You can only assess it using a re-measure if the PM and Contractor agree to use rates and lump sums as a basis - see last sentence of 63.13.
Bill rates break down as an accurate means of assessing change as they assume that costs are proportional to quantity which is often not the case : research has shown that typically delay and disruption account for 2/3rds of the change in costs due to a change.