Assume you've made a relative-frequency distribution graph of the above returns, which you believe enables this data to be approximated with (modeled by) a Normal Density function. To use this Normal Density function to model future SPSM returns, which big assumption is most important?

Date12/31/9912/31/0012/31/0112/31/0212/31/0312/31/0412/31/0512/31/0612/31/0712/31/0812/31/0912/31/1012/31/1112/31/1212/31/1312/31/1412/31/1512/31/1612/31/1712/31/18Return22.180%0.270%4.190%38.310%16.520%25.600%27.330%31.530%21.430%10.500%0.790%14.120%16.340%9.810%14.830%2.430%2.670%38.250%12.500%32.650% Date12/31/9912/31/0012/31/0112/31/0212/31/0312/31/0412/31/0512/31/0612/31/0712/31/0812/31/0912/31/1012/31/1112/31/1212/31/1312/31/1412/31/1512/31/1612/31/1712/31/18Return22.180%0.270%4.190%38.310%16.520%25.600%27.330%31.530%21.430%10.500%0.790%14.120%16.340%9.810%14.830%2.430%2.670%38.250%12.500%32.650%