Standard Deviation

I have been working with a file for awhile, and I am trying to take a standard deviation of grouped data but I can't find a good way.

I have line item data (say patient by patient) and I need to count it by day

Then I need to take each day and find the standard deviation by day for a month.

So:

Patient encounter 1 - Day 1

Patient encounter 2 - Day 1

Patient encounter 3 - Day 1

Patient encounter 1 - Day 2

Patient encounter 2 - Day 2

Patient encounter 3 - Day 2

Patient encounter 1 - Day 3

Patient encounter 2 - Day 3

Patient encounter 3 - Day 3

Average is 3, and I would sum by day:

Count of Day 1 - 3 visits

Count of Day 2 - 3 visits

Count of Day 3 - 3 visits

The standard deviation would be 0.

Thanks.