I have a set of historical monthly returns of a risk-free asset. Based on my calculations, I used stdev() to get the average monthly s.d. Then I multiplied the monthly s.d. with 12^0.5 to get the annual s.d of 0.61%

I am thinking shouldn't the s.d. of a risk-free asset be 0? Because the question I need to answer only considering the past period of the data, so should I use the 0.61% I calculated from the historical data as the average annual s.d. or use 0?

The monthly return from 1995-2012 period

0.00481 0.00478 0.00471 0.00473 0.00456 0.00452 0.00450 0.00440 0.00440 0.00447 0.00428 0.00417 0.00403 0.00413 0.00413 0.00418 0.00424 0.00429 0.00421 0.00424 0.00416 0.00419 0.00409 0.00419 0.00418 0.00428 0.00430 0.00421 0.00411 0.00421 0.00428 0.00413 0.00414 0.00428 0.00430 0.00420 0.00424 0.00419 0.00413 0.00417 0.00415 0.00413 0.00408 0.00384 0.00330 0.00368 0.00366 0.00362 0.00370 0.00370 0.00358 0.00375 0.00381 0.00379 0.00393 0.00390 0.00405 0.00423 0.00433 0.00443 0.00463 0.00474 0.00472 0.00483 0.00474 0.00497 0.00508 0.00500 0.00509 0.00514 0.00481 0.00429 0.00407 0.00368 0.00323 0.00302 0.00291 0.00293 0.00280 0.00220 0.00180 0.00156 0.00141 0.00138 0.00144 0.00149 0.00143 0.00144 0.00142 0.00140 0.00135 0.00136 0.00132 0.00103 0.00099 0.00098 0.00098 0.00094 0.00094 0.00089 0.00077 0.00075 0.00079 0.00078 0.00077 0.00078 0.00075 0.00073 0.00078 0.00078 0.00078 0.00085 0.00106 0.00111 0.00123 0.00138 0.00147 0.00173 0.00183 0.00194 0.00212 0.00228 0.00232 0.00237 0.00248 0.00268 0.00287 0.00285 0.00309 0.00323 0.00324 0.00353 0.00369 0.00376 0.00383 0.00393 0.00399 0.00413 0.00413 0.00401 0.00410 0.00412 0.00404 0.00415 0.00419 0.00412 0.00406 0.00394 0.00384 0.00402 0.00350 0.00324 0.00325 0.00273 0.00250 0.00229 0.00177 0.00105 0.00108 0.00144 0.00155 0.00136 0.00143 0.00094 0.00056 0.00016 0.00003 0.00011 0.00025 0.00018 0.00013 0.00015 0.00015 0.00015 0.00014 0.00010 0.00006 0.00004 0.00004 0.00005 0.00009 0.00013 0.00013 0.00013 0.00010 0.00013 0.00013 0.00013 0.00011 0.00012 0.00012 0.00013 0.00011 0.00008 0.00005 0.00003 0.00003 0.00003 0.00002 0.00001 0.00002 0.00001 0.00001 0.00003 0.00008 0.00007 0.00007 0.00008 0.00008 0.00008 0.00008 0.00009 0.00008 0.00008 0.00006 0.00006

I got monthly s.d. 0.00177 from =stdev.s() in excel

and the annual s.d. 0.00613 from 0.00177*12^0.5