calculating standard deviation of Completion Time

[berryandcherry6]

I am trying to calculate standard deviation of subject Completion time.I am using combo chart where SD should be a line graph.

How could i calculate standard deviation ,Below i have attached qvf file.Please help me in this regard.

[Gysbert_Wassenaar]

I don't see a Completion Time field in your data.

In general the standard deviation is calculated using the stdev function: stdev(MyField)

[berryandcherry6]

Here Completion Time is time_interval mentioned, which is taken as dimension.

Yes,if SD is calculated directly for each value at that time it will be helpful,but i need it to be found by taking count of user's into account.I mean i need to put a formula which contains number of users to calculate SD for each time_interval.