If you use published four-figure tables and double the figure P(0 Z 1) = 0.3413 given there, you will get the incorrect answer (often quoted in texts) of 0.6826. The reason for the error is that doubling a rounded decimal also doubles the rounding error: to five figures, P(0 Z 1) = 0.34134, and thus doubling it gives 0.68268 0.6827.