Managing a system where all patients get their first definitive treatment within 18 weeks of GP referral will sort the mean from the lean thinkers.

Mean thinkers run their departments or organisations in isolation. Their objective is to get more through for less cost and they worship the god of usage. Their patients wait for every department they have the misfortune to pass though. Why? Because to get 100 per cent usage mean managers keep a waiting list to feed any unused or empty slot that may come up. The mean manager sees a waiting list as an asset and does not have to account for the management cost of the list or the clinical consequences of any delay.

Lean thinkers see the world as processes in which patients, their information and money flow from department to department. Their god is the god of flow. Waiting lists are an indication that something is wrong. They arise not out of a lack of resource, but from mismatches between the variations in demand and the variations in capacity.

With lean thinkers, when the requests each day are greater than the clinic slots available that day, then the waiting list develops, but when the demand is less than the clinic slots, the slots are wasted. We also know most of the variation comes from our capacity, not the demand that we always blame.

To have no waiting list we need extra capacity to meet the waves of variation or to reduce the variation.

Then there is a simple trade-off between the management costs of the waiting list and the cost of reducing the variation. So how much does the waiting list cost?

If there is very short list, then a patient may not be able to book a clinic appointment on the exact day they request but for a few days later. This won't lead to any fuss or additional cost other than a few extra moments on the phone. However, a long waiting list incurs huge management costs, and error. Patients will return to their GPs to expedite or find out what is happening, clinicians will be prioritising and reprioritising patients, and senior managers will be disrupting meetings, clinics and operating lists to force long-waiting patients in.

In other words the cost of a waiting list grows in exponential, not linear, fashion as the list gets longer. The only cheap waiting list is no waiting list.

So where have we got to in meeting the 18-week target sustainably? Many NHS managers believe they can hit it by just doing more - probably by incurring additional cost in the private sector. Some are not yet ready to give up on the god of usage and are playing a dangerous game of "optimising the waiting list" for example, aiming for a six-week outpatient waiting time, a six-week wait for investigations, six weeks for procedures, and so on.

Very few are prepared to think lean and eliminate the waste of waiting lists completely, thus making the system both safer and cheaper for all patients.