how to calculate pooled standard deviation in MS Access

I have a question on how to calculate pooled standard deviation in MS Access. I know MS Access can use StDev(table.field) to get standard deviation. but how can I get pooled standard deviation such as subgroup size is 5? Take the following data as example, in table, there is a field "Measure" as following, set subgroup size as 5
Measure      subgroup
149.5308      1
149.5332      1
149.5288      1
149.5296      1
149.529              1
149.5323      2
149.5301      2
149.5317      2
149.5295      2
149.5328      2
149.5324      3
149.536              3
149.5306      3
149.541              3
149.5416      3
with measurement continue, the data will be continuously recorded. How to get pooled standard deviation so that I can get Cpk? Thanks.