Over the last four years, the common stock of Plymouth Shippers has had an average return of $10\mathrm{.}4$ percent. Three of those four years produced returns of $16\mathrm{.}1$ percent, $15\mathrm{.}6$ percent, and $9\mathrm{.}4$ percent, respectively. What is the standard deviation for this four$-$year period? $($Please keep six decimals. Answer should be in the format of $0\mathrm{.}000001.)$