We do not use a "global" CTR model to estimate how much traffic is coming from a keyword based on the target URL/domain's position in the SERP. 

Neither do we use a dozen of different models based on different SERP patterns. 

Here at Ahrefs, we model the CTR curve for every single keyword separately. 

In order words, we have a custom CTR curve for every keyword (based on clickstream data) and we apply the unique CTR (based on the custom CTR curve) for the keyword based on the target URL/domain's position in the SERP. 

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