Which of the following is not a proxy measure of the incubation period of a disease?

Communicable period is defined as the time during which an infectious agent may be transferred from an infected person or animal to another person or animal. As a rule, infectious diseases are not communicable during the incubation period, with a few exceptions. Latent period is equivalent to incubation period in non-infectious diseases. Serial interval and generation time are roughly equal to the incubation period. Ref: Park's Textbook Of Preventive And Social Medicine, By K. Park, 19th Edition, Page 93, 94.