# 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