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Student’s t-test for equal means

October 29th, 2008 No comments

If you do not wish to enter complex formula in Excel and you already have calculated average, count, and variance for your samples(Tip:use a PivotTable), then you can use these user-defined functions to calculate the t-test stat value and degrees of freedom required to do hypothesis testing. Both the functions provide an optional argument for the assumption of equal variances; by default it is set to false.

Here’s the code for t-test:
‘—————————————————————————————
‘ Procedure : TTESTM
‘ DateTime  : 10/29/2008 08:35
‘ Author    :
‘ Purpose   : To get the t-stat value when comparing two means with an optional input
‘               for equal or unequal variances
‘               by default the function assumes that the user is comparing means with unequal variances
‘—————————————————————————————
‘
Public Function TTESTM(davg1 As Double, dcnt1 As Double, dvar1 As Double, davg2 As Double, dcnt2 As Double, dvar2 As Double, Optional blnEqVar As Boolean = False) As Double

On Error GoTo TTESTM_Error
Dim dResult As Double, dNumer As Double, dDenon As Double, dPooledVar As Double

dNumer = davg1 – davg2

‘if equal variances are not assumed
‘http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm
If Not blnEqVar Then
    dDenon = Sqr((dvar1 ^ 2 / dcnt1) + (dvar2 ^ 2 / dcnt2))
Else
‘if equal variances are assumed, then calculated the pooled variance
    dPooledVar = Sqr((((dcnt1 – 1) * dvar1 ^ 2) + ((dcnt2 – 1) * dvar2 ^ 2)) / (dcnt1 + dcnt2 – 2))
    dDenon = dPooledVar * Sqr((1 / dcnt1) + (1 / dcnt2))
End If

dResult = dNumer / dDenon

TTESTM = dResult
Exit Function

TTESTM_Error:
TTESTM = Null

End Function

Here’s the code for the degrees of freedom:
‘—————————————————————————————
‘ Procedure : DOFTTESTM
‘ DateTime  : 10/29/2008 08:50
‘ Author    :
‘ Purpose   : To get the degrees of freedom for the t-test when comparing two means with an optional input
‘               for equal or unequal variances
‘               by default the function assumes that the user is comparing means with unequal variances
‘—————————————————————————————
‘
Public Function DOFTTESTM(dcnt1 As Double, dvar1 As Double, dcnt2 As Double, dvar2 As Double, Optional blnEqVar As Boolean = False) As Double

Dim dResult As Double, dNumer As Double, dDenon As Double

On Error GoTo DOFTTESTM_Error
‘if equal variances are not assumed, then use a complicated formula to compute degrees of freedom
‘http://www.itl.nist.gov/div898/handbook/eda/section3/eda353.htm
    If Not blnEqVar Then
        dNumer = ((dvar1 ^ 2 / dcnt1) + (dvar2 ^ 2 / dcnt2)) ^ 2
        dDenon = ((dvar1 ^ 2 / dcnt1) ^ 2 / (dcnt1 – 1)) + ((dvar2 ^ 2 / dcnt2) ^ 2 / (dcnt2 – 1))
        dResult = dNumer / dDenon
    Else
        dResult = dcnt1 + dcnt2 – 2
    End If

DOFTTESTM = dResult

Exit Function

DOFTTESTM_Error:
DOFTTESTM = Null

End Function

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Categories: String Operations, Uncategorized, Useful Procedures Tags: excel, UDF, VBA

Convert Text to Uppercase

September 17th, 2007 2 comments

If you want to convert the text to uppercase, use the following code; however, I recommend downloading ASAP Utilities, it has many functionalities, including text conversion. It doesn’t offer source code though.

Here’s my code for uppercase conversion:
(If you want to convert your text to lowercase, replace Ucase with Lcase function in the code)

‘Will convert selected range to Upper case, using array
Sub Conv2UCase()
On Error GoTo Conv2UCase_Error

Dim vDataArr As Variant
Dim lUpperBndRow As Long, lUpperBndCol As Long
Dim lRow As Long, lCol As Long

’store selected values in an array
vDataArr = Selection
‘get the upper bound of rows
lUpperBndRow = UBound(vDataArr, 1)
‘get the upper bound of cols
lUpperBndCol = UBound(vDataArr, 2)

‘Start a loop to go through all the elements of the array
For lRow = 1 To lUpperBndRow
    For lCol = 1 To lUpperBndCol
        ‘Check if the value is text, if not don’t convert
        If WorksheetFunction.IsText(vDataArr(lRow, lCol)) Then
            ‘Convert values to upper case
            vDataArr(lRow, lCol) = UCase(vDataArr(lRow, lCol))
        End If
    Next lCol
Next lRow
‘Return the converted values to the range
Selection = vDataArr
Exit Sub

Conv2UCase_Error:
MsgBox “Error ” & Err.Number & ” (” & Err.Description & “) in Sub:Conv2UCase”
End Sub

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Categories: String Operations, Uncategorized, Useful Procedures Tags: excel, UDF, VBA

A function to reverse a string

August 16th, 2007 1 comment

Very simple, uses the VBA function StrReverse to reverse the input string.

‘A function to reverse a string provided as input
‘For example, the string “abcd” will become “dcba”
‘Uses the VBA function StrReverse
Public Function ReverseString(sInputString As String) As String
On Error GoTo ReverseString_Error

ReverseString = StrReverse(sInputString)
Exit Function

ReverseString_Error:
ReverseString = “#ERROR#”
End Function

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Categories: String Operations, Useful Procedures Tags: excel, UDF, VBA

Concatenate function

July 24th, 2007 No comments

Oh, man, I can’t tell how useful that concatenate function is.

One repetitive use I found is to create OR/AND conditions for Access queries. I copy-paste the field values of a column from Access, do some filtering and my conditions are ready. Then I use this concatfunc to create a string to use in my Access query.

For example, look at this print screen:

The Range A1:A4 houses the string condition I want to use in my Access query to restrict the fruits from my data. In cell B1 I have the formula

=PERSONAL.XLS!concatfunc(A1:A4,CHAR(34) & ” or ” & CHAR(34))

, and the return string from this function is listed in cell B1.

Now, all you have to do is copy and paste this in Access criteria and put a quotation mark at the start and at the end of this string.

I have found one more use of this when I want to store some values in an Array, using the Array function in VBA.

One more print screen:

In this example, I insert a comma (CHAR(44) instead of string OR, and this function returns a string that I can use in VBA to store these values in an array using Array function, after adding a quotation mark, of course, at the start and at the end.

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Categories: String Operations, Useful Procedures Tags: excel, UDF, VBA

Range Concatenation with a character

June 29th, 2007 No comments

Are you frustrated that you have to concatenate a range, and you have to do that using CONCATENATE formula by entering each cell and typing a comma after every cell? Well, here’s a solution to it. A procedure or a function whatever you like. If you use the procedure, it allows you to choose the input range, concatenate character, and the output range. If you use the function, then you can enter the optional concatenate character (by default it is a comma (,)), and the input range.

Here are both:

Procedure:

Public Sub ConCatwChar()
Dim sChar2bAdded As String, rngRng2bCated As Range, sOutput As String, rngTarget As Range, c As Range
On Error GoTo ConCatwChar_Error
‘You could use this line to return the concatenated string in this cell
‘Set rngTarget = ActiveCell

Set rngRng2bCated = Application.InputBox(prompt:=”Select the range you’d like to concatenate with a charcter”, _
Title:=”Select Range”, Type:=8)

If rngRng2bCated Is Nothing Then Exit Sub

‘You could use this line to set the default to a comma and remove the inputbox line
’sChar2bAdded = “,”
sChar2bAdded = InputBox(“Enter the character you’d like to add between other cells”, “Enter Character”, “,”)

Set rngTarget = Application.InputBox(prompt:=”Select the range you’d like the output”, _
Title:=”Select Range”, Type:=8)

For Each c In rngRng2bCated
sOutput = sOutput & c.Value & sChar2bAdded
Next c

sOutput = Left(sOutput, Len(sOutput) – Len(sChar2bAdded))
rngTarget = sOutput

On Error GoTo 0
Exit Sub

ConCatwChar_Error:
MsgBox “Error ” & Err.Number & ” (” & Err.Description & “) in procedure ConCatwChar”
End Sub

Here’s the function:

‘You can change the option string character to nothing “” so that you get concatenated string without a character in between
Public Function ConCatFunc(rngRng2bCated As Range, Optional sChar2bAdded As String = “,”) As String
Dim sOutput As String, c As Range

On Error GoTo ConCatFunc_Error

For Each c In rngRng2bCated
sOutput = sOutput & c.Value & sChar2bAdded
Next c
sOutput = Left(sOutput, Len(sOutput) – Len(sChar2bAdded))
ConCatFunc = sOutput

On Error GoTo 0
Exit Function

ConCatFunc_Error:
ConCatFunc = “#Error#”

End Function

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Student’s t-test for equal means

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