Moving Range (specifically for IMR Chart)

[#1]

Hello,

I have been trying to develop an I-MR control chart.  I've found several useful threads, but am still having an issue.  I have my data in a table that is similar to the below:

Wave	Candidate	Date Trained	Date Certified
2014 Wave 1	A	3/20/2014	10/15/2014
2014 Wave 1	B	3/20/2014
2014 Wave 1	C	3/20/2014	12/16/2014
2014 Wave 1	D	3/20/2014	11/25/2014
2014 Wave 2	E	6/5/2014	5/7/2015
2014 Wave 2	F	6/5/2014
2014 Wave 2	G	6/5/2014	12/16/2014
2014 Wave 2	H	6/5/2014	11/25/2014
2015 Wave 1	I	2/5/2015	8/20/2015
2015 Wave 1	J	2/5/2015	7/13/2015
2015 Wave 1	K	2/5/2015	9/17/2015

My data points are the days between the two dates, so I am actually looking for the moving range between the days.  However, I have to have the blank cells as those candidates have not certified.  Is there a way to calculate the moving range of just the data that has a date in 'Date Certified' and ignore the null cells?

I have tried this several different ways, but cannot get it to work.  What I want is similar to the below:

Days to Certify	Range
209
271	62
250	21
336	86
194	142
196	2
158	38
224	66

I would appreciate any help with this.  When I try I either get a blank or a very large value because it is calculating the blank cells as well.

Thanks!

Ali

[Gysbert_Wassenaar]

See attached example.

[#3]

Thank you Gysbert.  I tried your solution and had two issues.  First, using just the 'Days to Certify' expression, no values populated in the table.  Then, when I added the 'Range' expression I got an error in expression of ')' expected.  When I put in the 'Days to Certify' as part of the 'Range' expression, the 'Days to Certify' is blue instead of red.  Could this be part of my issue?

Thanks for your help!