How to tell prophet to not forecast negative values

Question

using facebooks 'Prophet' package in R for forecasting. Was wondering if there is a way use it, such that it does not forecast negative values?

Thanks!

Answer 1

This is explained in the documentation in this page.

In brief, you should use "logistic" rather than "linear" growth. You must set a cap (a maximum logically possible value), and you can set a floor (if you don't set it, it will default to zero).

Assuming you have in df your data (a ds column with dates, and a y column with values).

You need to set a cap, for the past, as well as the future.

(I'm taking the 8.5 example figures included in the documentation... the value can change for each day, this should obviously be set to something meaningful for your case)

You can (but you don't need to) set a floor, for a minimum value.

df$cap <- 8.5
m <- prophet(df, growth = 'logistic')
future <- make_future_dataframe(m, periods = 1826)
future$cap <- 8.5
#future$floor <- 1.5
fcst <- predict(m, future)
plot(m, fcst)