# Method to show a time series of ~300 values is not stationary

I have a data series with approximatelly 300 values. I want to show that it is not weakly stationary, that is, show as mathematically as possible that its mean or its standard deviation vary with time. What would you recommend? I would like to be capable of implementing it on excel.

• What did you do finally? Would be glad to know. Oct 5, 2018 at 5:54

E.g. the variance over the period $t:t+n$ with the one over $t+\ell:t+n+\ell$, where $n\in(0,299-t-\ell]$ is the MV window size, $\ell\in(0,299-t-n]$ is the time distance between the two compared periods (possibly overlapping, but imperfectly), and $t\in[0,299−n-\ell]$. An illustration below