Difference between two values by Week

// That's pretty easy in fact if you have the
// right structure. The table that stores
// weeks must have a column that is a contiguous
// sequence of integers. Let's name the column
// WeekID. Normally, such a column would be hidden
// as it's just a key for the weeks to make them
// distinguishable from each other. This table
// will be connected to some fact table with
// measures.

[Week-by-Week Delta] =
IF(
	// When we calculate the delta,
	// we have to make sure that
	// only one week is visible in
	// the current context.
	HASONEVALUE( Weeks[WeekID] ),
	
	var __currentWeekId = SELECTEDVALUE( Weeks[WeekID] )
	var __currentValue = [CountResult]
	var __priorValue =
		CALCULATE(
			// This is the base measure
			// of which you want to calculate
			// the delta.
			[CountResult],
			Weeks[WeekID] = __currentWeek - 1
		)
	var __delta = __currentValue - __priorValue
	return
		__delta
)

[Increase/Decrease] = DIVIDE( [Week-by-Week Delta], [CountResult] )

You might need to adjust the measure to deal with boundary conditions. Simply test it and fix the places where it goes astray.

Best

D