1.2 Right censoring

When an individual is lost to follow-up, we say that she is right censored , see Figure 1.2.

FIGURE 1.2: Right censoring

As indicated in Figure 1.2, the true age at death is T, but due to right censoring, the only information available is that death age T is larger than C. The number C is the age at which this individual was last seen. In ordinary, classical regression analysis, such data are difficult, if not impossible, to handle. Discarding such information may introduce bias. The modern theory of survival analysis offers simple ways to deal with right censored data.

A natural question to ask is: If there is right censoring, there should be something called left censoring, and if so, what is it? The answer to that is that yes, left censoring refers to a situation where the only thing known about a death age is that it is less than a certain value \(C\). Note carefully that this is different from left truncation, see the next section.