Table 1: Intermediate end points of clinical trials and potential biases.
| Intermediate end point | Potential bias |
| Improved cancer-related survival in the screened population | Lead time bias: In reality the survival of the screened population has not been prolonged; only the duration of time during which they have been known to have cancer has been prolonged |
| Screening lead to diagnosis of cancer at an early stage | Length time bias: The cancers diagnosed through screening are less aggressive than those diagnosed when the patient is symptomatic. In other words, some of the most aggressive cancers escape screening, as they emerge in the interval during the screening sessions |
| Screening leads to the diagnosis of more cancer | Over-Detection bias: most of the cancer diagnosed through screening may not become clinically relevant during a person’s lifetime. This type of bias is of particular concern to older individuals with limited life expectancy. |