[MUSIC] So, we have discussed the definition of screening. We review what is the natural history of disease. Now to understand really screening, we have to put together prevention and the natural history of disease that we just discussed and this is what we will review in this lesson. The amount of time by which the diagnosis is early in screening is called lead time. Lead time is necessary in screening. You have to make a diagnosis earlier. You have to treat the disease earlier. You will often see lead time discussed as a bias, something which is a systematic error, but always remember that lead time is necessary. Still, you have to avoid the lead time bias that will be discussed in the module or lessons on bias in screening. But lead time is really necessary, is fundamental in screening. If you do not capture the disease earlier, if you do not have a treatment that is more beneficial early than later, you do not have any reason to implement screening whether at population level or or at an individual level. Again, lead time is the time that diagnosis would have occurred if screening had not been done. This is what you have done earlier than usual care. Sometime, it works. Most of the time, you do not change the prognostic of the disease by identifying earlier the disease. We will discuss the reason why. On this slide again, this concept will be discussed in other lessons. But you have to remember that lead time will give you the advantage of improving the prognostic. And as you can see on this slide, this is only worth doing when you can really have an impact on the mortality of the population. So for example, on this slide, you have three situations. Each are characterized by the natural history of disease that we discussed. The first, molecular changes due to risk factor exposure. Early diagnosis by screening. Usual diagnosis, because of presence of signs or symptoms and death. Let's take an example of the absence of screening. The natural history would be that at some point in your life, individual in the white circle will have signs, symptoms and the diagnose will be post. At a given time, this individual will die. And ineffective screening will be the situation where you use the test, screening test for early diagnosis. And you can see here, the lead time, the time you have won by using a screening test. But as you can see by knowing earlier, even by treating earlier, this individual in the black circle will die at the same moment or same time as the individual in the absence of screening. So for him or for the patient, for the participant, there is no gain. Maybe he has known his disease for two, three, four years before you would have known. But maybe even taking advantage of it being more anxious, being more concerned about this disease result having any benefits of it. An effective treatment, this is the last scenario is the individual in blue circle who has gain some time. But the lead time and thanks to that lead time, The earlier diagnosis, the earlier effective treatment. As you can see, there's a real impact on his life or her life. Because he would live longer as shown on this figure. We will discuss in the next lesson the lead time, how lead time can lead to a over diagnosis and how it is related. [MUSIC]