Totaling Samples per Month based on an Average

[sbarrett]

I have a data set where individual samples failures from particular lots are recorded as individual rows.  Each failure has partilar columns for lot#, severity, sample size, and month.  I would like to add a column for total samples tested per month.  I cannot simply sum the samples sizes because there are sometimes multiple rows entered per lot.  Instead, I must average the sample sizes per lot and then sum those averages.  I was thinking of using the cross tab and the multi-field tool but I am having trouble with the correct expression to use.  Can someone offer some advice?   below is an example of what I am looking to do.  

This is what i currently have:

Lot	Sample Size	Severity	Test Month
A1	400	A	Jan
A1	400	B	Jan
A1	200	N/A	Jan
A2	200	N/A	Jan
A3	200	B	Feb
A3	200	B	Feb
A3	200	B	Feb
A4	200	B	Feb
A5	200	N/A	March
A6	200	B	March
A6	200	B	March
A7	200	N/A	March

This is what I look like to have:

Lot	Sample Size	Severity	Test Month	Total Samples per Month
A1	400	A	Jan	600
A1	400	B	Jan	600
A1	400	N/A	Jan	600
A2	200	N/A	Jan	600
A3	200	B	Feb	400
A3	200	B	Feb	400
A3	200	B	Feb	400
A4	200	B	Feb	400
A5	200	N/A	March	600
A6	200	B	March	600
A6	200	B	March	600
A7	200	N/A	March	600

[Joe_Mako · Accepted solution]

How about the attached workflow?

It uses Unique to select a single record per Lot-Month combination, then Sums Sample Size to the Month granurality, and joins the aggregrated value back in with your original data.

[sbarrett]

This is perfect! Thank you!