Calculating grouped variance from a frequency table in R

Question

How can I, in R calculate the overall variance and the variance for each group from a dataset that looks like this (for example):

Group Count Value
A      3              


        

Answer 1

Here's a quick wrapper with base R. First step is to grow your data set by Count, and then calculate variance by group

df1 <- df[rep(seq_len(nrow(df)), df$Count), ]
with(df1, tapply(Value, Group, var))
#   A   B 
# 2.7 4.0 

Or similarly

aggregate(Value ~ Group, df1, function(x) c(Var = var(x), Count = length(x)))
#   Group Value.Var Value.Count
# 1     A       2.7         5.0
# 2     B       4.0         4.0