All of the following are used as proxy measure for incubation period except-

Incubation period is the time interval between invasion by an infectious agent and appearance of 1st sign or symptom of disease whereas the period of communicability is the time during which an infectious agent may be transferred directly or indirectly from an infected person to another. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 107, 108