From observed event rates to event rate per anno and 5-year risk

Dear all,

I try to figure out how the Cholesterol Treatment Trialists’ (CTT) Collaboration calculated their meta-analysis but I have some trouble to reproduce the results.

According to the Baseline characteristics table and Figure 1 of the paper (Lancet. 2010 Nov 13;376(9753):1670-81.):

  • 21 trials comparing statin with control were analyzed

  • the median follow-up was 4.8 years

  • the number of events / patients in the control group was 8934 / 64 782

  • the number of events / patients in the statin group was 7136/64 744

Dividing the number of events by the number of patients per group leads me to an event rate of 11.02% and 13.79% for the control and statin group, respectively.

Maybe it is naïve (assuming a continuous and linear relationship between event occurrence and time), but I then divided these rates by the median follow-up of 4.8, leading to event rates per anno of 2.87% and 2.30%, respectively.

However, the authors depicted per anno rates of 3.6% and 2.8%, respectively.

Moreover, the unweighted risk ratio calculated based on the above-mentioned numbers is 0.80 and not 0.78 as the depicted subtotal treatment effect in Figure 1.

Most likely, I am just naïve and my statistical calculations are wrong. Therefore, I would be happy to get some expert support/guidance here!

In the slide deck provided on the CCT website, the authors present the 5-year risk of an event. Is it possible to calculate this measure based on the published data (without individual patient data)?

I am looking forward to your replies.

but you didn’t follow the equation given in their stats section, typical for meta-analysis: “log of the rate ratio per mmol/L (log RR) is calculated as S/V with variance 1/V (and hence with 95% CI of S/V±1·96/√V), where S is the sum over all trials of d (o–e) and V is the sum over all trials of d2v”. You can maybe find an online calculator that does the calculation. Although it can be difficult to reproduce exactly because of rounging error

link to article