problem with a calculation

HI Giada90 ,

The solution that you have accepted, will only work when you have only 2 categories in your data - A and B.

If you are interested in a dynamic solution for your problem, say where you have more number of categories than just A and B, then try the follwing solution I have created.

The sample data I am considering is:

First thing I did was created a Rank column at the type column level  in the data using the following DAX expression:

Rank = 
RANKX(
    TypeTable, 
    TypeTable[total value], , 
    DESC, Dense
    )

When you move this to table visual you will see basically a Rank assigned for every type value in the data.

Now the next thing is we need to get the sum of value column at a Type level.

This we can get using following DAX:

total value at Type level = 
CALCULATE(
    SUM(TypeTable[value]),
    ALLEXCEPT(TypeTable,TypeTable[type])
)

Now we need a difference between these values which should be 31 - 12 = 19 in this case as we just have 2 categories in TYPE column. The following DAX will give the required answer:

Difference at Type level = 
var minRank = CALCULATE(MIN(TypeTable[Rank]), ALL(TypeTable))
var totalValue = CALCULATE(
    SUM(TypeTable[value]),
    ALL(TypeTable),
    TypeTable[Rank] = minRank
)
var nextTotal = 
CALCULATE(
    SUM(TypeTable[value]),
    //ALLEXCEPT(TypeTable, TypeTable[type]),
    ALL(TypeTable),
    TypeTable[Rank] = minRank + 1
)
var diffVal = totalValue - nextTotal
RETURN
diffVal

The result is as follows:

As part of best practice, I always try to use the dynamic solutions as data can change over time.

Hope this solution helps.