So, on hearing Matthew Thompson open up a mini-session with natural frequencies my mind turned to the healing power of crystals, and I become acutely concerned that the open-minds approach of the Teaching EBM Conference had gone too far.

But this was quashed quickly by his description: ‘Natural frequencies’ means ‘real numbers’ … like 30 of 100 patients, not 30% or 0.3 … and this is easier for most people to deal with.

He went on to show that a single box can crack the nut of turning diagnostic test information from meaningless prattle into something highly understandable:

Using this approach means you can loose the faff of nomgrams, converting into odds or pocket calculating and work out, really quickly, what a test result means.

It works like this: take a problem where 30% of the walk-ins will have it. The test you run has 50% sensitivity and 90% specificity. Of 100 people, 30 have the condition.  50% sensitivity means half of the diseased have a +ve test (= 15). 90% specificity means 9/10th of the undiseased will have a -ve test … so one-tenth (=7) will be test +ve.

The result – 70% of the positive tests (15/22) really do have the condition.

Re-run it with a test prevalence of 60% :

60 >>>>>  30
/                     .
/disease               .
100                                .
\not disease         .
\                    .
40 >>>>>    4

Now 30/34 ~ 90% of +ve test results are in folk who really have the disease.

Try it yourself with a prevalence of only 10% and see what the positive test really means …