LinRegSlope (MDX)
Calculates the linear regression of a set, and returns the value of the slope in the regression line, y = ax + b.
 Set_Expression

A valid Multidimensional Expressions (MDX) expression that returns a set.
 Numeric_Expression_y

A valid numeric expression that is typically a Multidimensional Expressions (MDX) expression of cell coordinates that return a number that represents values for the yaxis.
 Numeric_Expression_x

A valid numeric expression that is typically a Multidimensional Expressions (MDX) expression of cell coordinates that return a number that represents values for the xaxis.
Linear regression, that uses the leastsquares method, calculates the equation of a regression line (that is, the bestfit line for a series of points). The regression line has the following equation, where a is the slope and b is the intercept:
y = ax+b
The LinRegSlope function evaluates the specified set against the first numeric expression to obtain the values for the yaxis. The function then evaluates the specified set expression against the second numeric expression, if specified, to get the values for the xaxis. If the second numeric expression is not specified, the function uses the current context of the cells in the specified set as the values for the xaxis. Not specifying the xaxis argument is frequently used with the Time dimension.
After obtaining the set of points, the LinRegSlope function returns the slope of the regression line (a in the previous equation).
Note: 

The LinRegSlope function ignores empty cells or cells that contain text or logical values. However, the function includes cells with values of zero. 
Example
The following example returns the slope of a regression line for the unit sales and the store sales measures.
LinRegSlope(LastPeriods(10),[Measures].[Unit Sales],[Measures].[Store Sales])