Torubles on calculating average resolution time

[#1]

Hi,

I am trying to create a simple pivot table to calculate the average resolution time of all closed incidents but I'm not getting the expected results.

I have a table with the following fields:

Incident Code

Date Closed (its null when it has not been closed yet)

Date opened

What I'm doing is to create a pivot table where I put as a dimention the "Incident Code" and I create the follwing Expression [Date closed] - [Date opened] and I call it Time for closing.

The result that I get would be the following as an example

Incident Code     Time for closing

1                         2,3

2                         1,2

3                         4,6

4                         -

5                         -

6                         3,2

My problem is that what I want to get is the average of all the incidents that have been closed. The problem is that when I put the dimention out it does not calculate. I've tried the following expretions but none of them is giving me good results:

1.  avg([Date closed] - [Date opened]

2. (if(isnull([Date Closed]) or IsNull([Date Opened]),0,1)*([Date Closed]-[Date Opened])) / (Count(Incident Code)- count(isnull(Date Closed) or isnull(Date Opened))

and other expressions but I'm getting nowhere. Does anybody know how should I proceed? Thanks in advance.