If you have this... how do you calculate that....?
 

If you have the standard deviation for one table per hour,

how do you calculate the standard deviation of 4 tables per hour?

I'm actually ashamed to ask, because I should know this.

Re: If you have this... how do you calculate that....?
 

Other things being equal, SD increases as the square root of the number of tables.

SD(4 tables)= 2*SD(1 table)

Re: If you have this... how do you calculate that....?
 

[ QUOTE ]
Other things being equal, SD increases as the square root of the number of tables.

SD(4 tables)= 2*SD(1 table)

[/ QUOTE ]

More generally, do the math with variances, not standard deviations. You square the standard deviation to get the variance, multiply it by four, and then take the square root to get the standard deviation of the four tables combined. As uuDevil points out, that's twice the standard deviation of the one table.

Obviously, uuDevil's method is simpler. But, if you have four tables with different standard deviations, you need to shift to the variances and back.