Private Means and the Curious Incident of the Free Lunch

We show that the most well-known and fundamental building blocks of DP implementations – sum, mean, count (and many other linear queries) – can be released with substantially reduced noise for the same privacy guarantee. We achieve this by projecting individual data with worst-case sensitivity $R$ onto a simplex where all data now has a constant norm $R$. In this simplex, additional ``free’’ queries can be run that are already covered by the privacy-loss of the original budgeted query, and which algebraically give additional estimates of counts or sums.