Paul Oginsky, chief executive of Personal Development Point, which helped develop the government's flagship National Citizen Service (NCS) initiative, said youth organisations need to change the way they calculate the value of their work.
"For many years, people in the youth sector have been looking for a definitive and universally accepted way of measuring the impact of their work," A white paper published by Oginsky, titled A Way Forward For Character Development: The missing piece of education, states.
"So many people have tried and so many methods have been proposed that this quest has become known as the ‘Holy Grail' of youth work.
"Let us kill this myth now (spoiler alert), there is no holy grail. There is no way of measuring impact on people that is definitive and universally accepted.
"There is no such thing as a unit of confidence, loyalty, honesty, motivation or any of the characteristics which this kind of work seeks to impact."
Different youth groups and organisations across the UK use impact measurements to demonstrate their worth - most notably the NCS programme, which publishes an evaluation report every year to measure the experience of participants as well as four impact areas that the programme seeks to address.
Instead, Oginsky said organisations have to concentrate on "assessing" impact by asking the person themselves to describe if and how they have changed and then to ask for witnesses verification by those who know them.
"Being assessed is not as reliable or replicable as being measured but it is the best approach for this kind of work," he said.
"It provides strong evidence of change and development in young people and can be conducted in ways which are rigorous.
"Some funders may still insist on measurable evidence for a wider impact - such as all young people having full and happy lives. This is a waste of everyone's time.
"Far more effective is to ensure programmes include robust methods to assess the progress made by young people, as evidenced by them and witnessed by others."
The white paper, which makes a number of recommendations for practitioners, commissioners and policy makers, has been published in a bid to encourage investment in character development opportunities for young people.