Ja. Jeg fandt det!

Dicte would be proud. I had been looking for causality to the mammography trial in the two Swedish counties of Dalarna and Ostergotland, but the culprit turned out to be Malmo, across the way from Denmak. Ironically, Malmo was fingered by lead author of the Two-County trial, Laszlo Tabar.

Tabar argued in the BMJ that the Malmo trial shouldn't be used to determine the value of mammography screening because between 70% and 74% of women invited to the trial actually had a mammogram and some 24% of women who were not invited had a mammogram.

Fik dig!

We can write down the following system of equations, where A is the probability of dying from breast cancer conditional on getting mammography screening and B is the probability of dying from breast cancer without mammography screening.

0.7*A + 0.3*B = 63/21088

0.24*A + 0.74*B = 66/21195

A = 0.00291

B = 0.00317

While the intent-to-treat analysis suggest a 4% reduction in death from breast cancer associated with mammographic screening. Accounting for the actual take up rate suggests an 8% reduction. Note that none of this accounts for sampling variation and is only presented for illustrative purposes.

Now this analysis also assumes that everyone who took up mammography screening is the same as those that didn't take up the screening, excepting for the fact that they took up mammography screening. This is plainly not true.

To be continued....

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