常见问题－ 请问Pooled StDev如何计算？

请多指教---
Two-sample T for a vs b
N Mean StDev SE Mean
a 6 18.19 6.00 2.4
b 6 18.01 4.42 1.8
Difference = mu a - mu b
Estimate for difference: 0.77
95% CI for difference: (-5.94, 7.49)
T-Test of difference = 0 (vs not =): T-Value = 0.26 P-Value = 0.802 DF = 10
Both use Pooled StDev = ( )
请问Pooled StDev如何计算？