A normally distributed quality characteristic is monitored through the use of an x bar and an R chart. These charts have the following parameters $($n $=4)$:

x bar Chart R Chart

UCL $=626\mathrm{.}0$ UCL $=18\mathrm{.}795$

Center line $=620\mathrm{.}0$ Center line $=8\mathrm{.}236$

LCL $=614\mathrm{.}0$ LCL $=0$

Both charts exhibit control.

Suppose an s chart were to be substituted for the R char. What would be the center line value of the s chart?