The Impact Valuation Statement measures overall improvements and does not directly identify the activities’ specific contribution to it. To give a better estimate of the activities' specific impact, an adjustment has been made that reduces the value by a percentage to try to account for the social improvements that would have happened anyway in the absence of the activities. This is known as a 'deadweight adjustment', and is made on the basis of the average deadweight for different categories of activities.
The model uses average deadweight figures from the HCA Additionality Guidance 2014 that acknowledges that a proportion of change would have happened without your intervention and thereby prevents over-claiming. Using the HCA figures prevents a less robust figure being used. If a comparison group were used, the HCA deadweight would not be applied as both methods seek to address the same problem. See Measuring the Social Impact of Community Investment: A Guide to using the Wellbeing Valuation Approach.