Excel has a function called stdev(Array) use close(t)/close(t-1) of over 100 days say then the function stdev (Of those fugures) GL
Arnaud, Your English is perfectly fine. Didn't realize from your first reply that it's you who is the author of that website. Really nice, precise, up-to-date work on volatilites, correlations, etc... appreciated by many, I'm sure. Welcome to ET!