The ‘generation time’ is the one that can’t be negative. Suppose Alice gets infected on day 1, infects Bob on day 2, Bob shows symptoms on day 3, and Alice shows symptoms on day 4. We end up with:
Incubation periods of 3 days (for Alice) and 1 day (for Bob)
Generation times of 1 day (Bob infected—Alice infected)
Serial intervals of −1 day (Bob symptoms—Alice symptoms)
The ‘generation time’ is the one that can’t be negative. Suppose Alice gets infected on day 1, infects Bob on day 2, Bob shows symptoms on day 3, and Alice shows symptoms on day 4. We end up with:
Incubation periods of 3 days (for Alice) and 1 day (for Bob)
Generation times of 1 day (Bob infected—Alice infected)
Serial intervals of −1 day (Bob symptoms—Alice symptoms)