vba average calculator userform - vba

Question

Hey im trying to write a small VBA program that calculates the average quiz score the problem is when i enter in the third number it comes back incorrect after doing the first two right.What am i missing?
This is the design view
Option Explicit
Dim total As Double
Dim number As Double
Dim average As Double
Private Sub CommandButton1_Click()
If IsNumeric(TextBox1.Value) = True Then
total = CDbl(average + TextBox1.Value)
number = CDbl(number + 1)
average = CDbl(total / number)
TextBox2.Value = number
TextBox3.Value = average
TextBox1.Value = ""
Else
MsgBox ("please enter a number")
TextBox1.Value = ""
End If
End Sub

Answer 1

Your math is off. If you want to do it, using the previous average, and the new value, then the formula you should use is this:
( [old average] + ( [new value] / [count - 1] ) ) / ( [count] / [count - 1] )
However, as you might see, this is not a very easy-to-read design. I would go for a design, using an array. That way you can keep the math a lot more intuitive, plus you can look back at old score, if you want. The easiest design is probably to keep the array the size of your score count. This requires redim() each time you add a new score. From a performance point-of-view, this is quite bad practice. But in your tiny calculator, thats hardly going to be a problem.

Answer 2

I think you should get number, average, and total value before you do the calculations:
Option Explicit
Dim total As Double
Dim number As Double
Dim average As Double
Private Sub CommandButton1_Click()
number = 0
average = 0
total = 0
If IsNumeric(TextBox1.Value) = True Then
total = CDbl(average + TextBox1.Value)
number = CDbl(number + 1)
'prevent error
If number > 0 then
average = CDbl(total / number)
Else
average = 0
End If
TextBox2.Value = number
TextBox3.Value = average
TextBox1.Value = ""
Else
MsgBox ("please enter a number")
TextBox1.Value = ""
End If
End Sub
Hope this help.