I know you made one mistake (C12 <> C11) so I'm going to post this, even though it doesn't agree with your results
Enum ColumnAndArrayIndices
Names = 1
Sizes
Forms
Sects
Mins
Maxs
Diffs
Times
TimeAvgs
End Enum
Sub CWB1021()
Dim Data_Set As Variant
Dim i As Long
Dim V1225DiffSum As Double
Dim V1225TimeSum As Double
Dim V1225Ave As Double
Dim V875DiffSum As Double
Dim V875TimeSum As Double
Dim V875Ave As Double
Dim C875DiffSum As Double
Dim C875TimeSum As Double
Dim C875Ave As Double
Dim L850DiffSum As Double
Dim L850TimeSum As Double
Dim L850Ave As Double
Dim L875DiffSum As Double
Dim L875TimeSum As Double
Dim L875Ave As Double
Data_Set = Sheets("Sheet1").UsedRange
For i = 3 To UBound(Data_Set)
If Data_Set(i, Sects) = "V" Then
If Data_Set(i, Sizes) = 1225 Then
V1225DiffSum = V1225DiffSum + Data_Set(i, Diffs)
V1225TimeSum = V1225TimeSum + Data_Set(i, Times)
ElseIf Data_Set(i, Sizes) = 875 Then
V875DiffSum = V875DiffSum + Data_Set(i, Diffs)
V875TimeSum = V875TimeSum + Data_Set(i, Times)
End If
ElseIf Data_Set(i, Sects) = "C" Then
If Data_Set(i, Sizes) = 875 _
And Data_Set(i, Forms) = Data_Set(i - 1, Forms) Then
C875DiffSum = C875DiffSum + Data_Set(i, Diffs)
C875TimeSum = C875TimeSum + Data_Set(i, Times)
End If
ElseIf Data_Set(i, Sects) = "L" _
And Data_Set(i, Forms) = Data_Set(i - 1, Forms) Then
If Data_Set(i, Sizes) = 850 Then
L850DiffSum = L850DiffSum + Data_Set(i, Diffs)
L850TimeSum = L850TimeSum + Data_Set(i, Times)
ElseIf Data_Set(i, Sizes) = 875 Then
L875DiffSum = L875DiffSum + Data_Set(i, Diffs)
L875TimeSum = L875TimeSum + Data_Set(i, Times)
End If
End If
Next
V1225Ave = V1225DiffSum / V1225TimeSum
V875Ave = V875DiffSum / V875TimeSum
C875Ave = C875DiffSum / C875TimeSum
L850Ave = L850DiffSum / L850TimeSum
L875Ave = L875DiffSum / L875TimeSum
With Sheets("Sheet1").Columns(TimeAvgs)
For i = 3 To UBound(Data_Set)
If Data_Set(i, Sects) = "V" Then
If Data_Set(i, Sizes) = 1225 Then
.Cells(i) = V1225Ave
ElseIf Data_Set(i, Sizes) = 875 Then
.Cells(i) = V875Ave
End If
ElseIf Data_Set(i, Sects) = "C" Then
If Data_Set(i, Sizes) = 875 _
And Data_Set(i, Forms) = Data_Set(i - 1, Forms) Then
.Cells(i) = C875Ave
End If
ElseIf Data_Set(i, Sects) = "L" _
And Data_Set(i, Forms) = Data_Set(i - 1, Forms) Then
If Data_Set(i, Sizes) = 850 Then
.Cells(i) = L850Ave
ElseIf Data_Set(i, Sizes) = 875 Then
.Cells(i) = L875Ave
End If
End If
Next
End With
End Sub