Even better quote from the Guardian:
Brexit vote has cost each household more than (pounds)600 a year, says NIESR
Further down:
The National Institute of Economic and Social Research (NIESR) said it is 'almost certain' the leave vote has damaged living standards and hit the growth potential of the economy. The thinktank also scaled back its expectations for growth in the UK for the next three years.

So the NIES are almost certain that the leave vote has damaged living standards, but they are able to quantify this (possible) effect by giving it a specific value.