Modules & VBA :: Select Case Function

My problem now is about evaluating the result of that query in order to calculate quarterly and annual performances. The query returns correctly:

1) NULL - when I don't have values in the given month
2) 0 - when one of the components of the performance formula is 0
3) value (positive or negative) which can include any positive or negative value

In order to calculate the quarterly values I need the performance of the last month in the quarter and the first month in the quarter. My monthly values are stored in columns so say for Quarter 1, I would need something like this:

Code:
[mar]/[jan]-1

As you might have thought, the problems come when I have NULL or 0 values. Let me give you a couple of examples.

[jan] is NULL
[feb] is a value
[mar] is a value

Than my formula would have been

Code:
[mar]/[feb]-1

Second case:

[jan] is 0
[feb] is 0
[mar] is 0

In this case I would like the formula return '-' because I want to be able to spot the case in which the quarterly performance is 0 because the initial values were 0 from the case in which the performance was actually 0 (i.e., say [jan] = 101 and [mar] = 101 then performance is 0 which is different from having [jan] and [mar] = 0 thus performance '-').You can combine the three states (NULL, 0, value) with the three months and find many combinations (I have found 27 relevant ones).

I initially thought to use SWITCH in SQL to evaluate the 27 combinations but I found out that SWITCH would evaluate non more than 15 conditions. So I guess the solution should be use a function in VBA which does what I was about to do with SWITCH in SQL.

I have built a test function which evaluates the numerator of my formula using only two cases (CASE 1: all variables have a value <>0, and CASE 2: the first month is null, the second is 0 and the third a value). Here the code:

Code:
Function evaluate_s(Var1, Var2, Var3) As Double
Select Case Var1
Case Is <> 0

[code]....

My main question here is what is wrong in the syntax of my function. Why is not correctly evaluating at least the two cases in my function? I always get 0 for all records.