Take randomly sample based on groups

Question

I have a df made by almost 50,000 rows spread in 15 different IDs (every ID has thousands of observations). df looks like:

        ID  Year    Temp    ph
1           


        

Answer 1

library(data.table) #1
df <- data.table(df) #2
df[,group_num := sample(2,.N,replace = TRUE,prob = c(500,.N-500)/.N),by = "ID"] #3
df_sample = df[group_num == 1,] #4

or you can change line #3 and #4 to:

df[,random_num := sample(.N,.N),by="ID"]
df_sample  = df[random_num <=500,]