Many people who suffer from irregular heartbeats (atrial fibrillation) which might cause stroke, need to take blood thinners to prevent it. It is important to prescribe the correct dose of blood thinners to the right patients to ensure the treatment works however avoiding complications. In the recent years, new blood thinners have been available; they require less laboratory tests and fewer visits to a doctor compared to older therapies. This study will look at how the general practitioners in the UK prescribe blood thinners according to the instructions given by the product manufacturer. We will use primary care data that is routinely collected by the general practitioners about their patients but without any possibility to identify individual patients. The results will help us to understand the magnitude of deviation from instructions in order to ensure that the patients benefit from the treatment.