Losing sleep over the "do-loop" in Seattle.

After close encounters in Seattle with Multisystemic Therapy and Functional Family Therapy, our study tourists have begun tussling with the concept of fidelity.

It has become routine for developers and practitioners on both sides of the Atlantic to stress the need to deliver programs exactly as they were designed to be delivered.

When the goal is to improve children’s health and development there can’t be much argument. Inconsistent delivery of effective programs invariably tends to eradicate their potential impact.

But for politicians and other policy makers, particularly those responsible for large children’s services systems, fidelity can smack of ideology or of a damaging impulse to limit the scope for negotiation in the relationship between clinician and client.

As one of our travelers put it: “We have to meet the particular needs of every individual child. If we’re not careful, we’re going to be forcing children and families into the slots inside evidence-based programs”.

Graham Allen, UK Member of Parliament for Nottingham North, captured similar misgivings when he suggested to Functional Family Therapy’s developer, Jim Alexander, “there’s a danger of fidelity becoming a sclerosis”.

What becomes clear from closer examination of both programs is that proven models have an inherent elasticity which permits flexibility in the relationship between clinician and client.

Multisystemic Therapy workers call it “the dox,” the "do-loop" that helps them agree with young people and their families the nature of the problem and its likely causes and to arrive at a potential solution. Diagnosis will vary from family to family, but the method for deciding it will be exactly the same.

Adherence did not mean applying the model in a lockstep way, Jim Alexander said.

The leaders of children’s services have learned to listen to the arguments as if with the ears of the people they manage. They will want to know, for example, what motivation a social worker, probation officer or mental health specialist is likely to have to learn how to deliver Multisystemic Therapy, Functional Family Therapy or any other behavior modification model.

Does even the best of the bunch merely capture and brand what a well-trained sympathetic practitioner would routinely do? In the end, doesn’t it all boil down to the quality of that central relationship between the clinician and the child or the clinician and the family?

Nobody knows. There is an emerging science but it does not yet have the power to say what it is in the relationship between practitioners and the people they support that makes a difference to the well-being of children.

And mindful of the limitations on the public purse, Graham Allen put it to Jim Alexander that he had designed a Ferrari when what policy makers needed was something more like a Model T Ford.

Alexander disagreed – and modernized the comparison. There was huge scope for development in the application of proven models. “We came right out of the stocks in 1971 trying to build a Toyota Camry,” he said.

Functional Family Therapy delivers around 30,000 units a year worldwide. Since they launched it in the US in 1983, Toyota have sold nearly seven million Camrys.

By that reckoning, there’s plenty of scope for more engineering, marketing and maintenance work before program developers and the systems and communities that buy their products can be compared to the automobile industry, sclerotic or otherwise.

• In today's video, Functional Family Therapy's creator Jim Alexander extends the Ferrari-Toyota comparison. Economy and usefulness win out.