Comparison between a week and its former week

[BernardBernoulli]

Hello.

i wanted to write a simple expression with set analysis which compares a field with a weeknumber per entry with its former week.
So basically I have a table like this:

Week | Article | Amount

11       |  item1 | 10

10       | item 1 | 20

Now i want to put out the difference between these two amounts in another column and wrote something like this:

{<Week= {Week}>}Amount - {<Week= {lastWeek}>}Amount

But this only puts out the difference to the former week if its a positive amount, I'd also like to know if the amount in week 11 was less than in 10, like above in the table, it's negative amount difference.

What am I missing?

(Please note that I created the "lastWeek" field in the load script as following:
[Week] - 1 as lastWeek

[TheoAtragie]

Hello,

You can try this :
sum({<Week= {Week}>}Amount) - sum({<Week= {lastWeek}>}Amount)

[BernardBernoulli]

Hey there,

thanks for the reply. 
I tried that before, but this will give me 0 values in every row though.