Here is the explanation and sample code to use a collection to
determine is an item exists in a list.

I wrote the notes below as aprt of my operations and coding
practices.  This is the first time they will have been read by someone
else.  I hope they make sense..

VBA does not have a method for quickly determining if a item is a
member of a list.  For example in an array of names does the array
contain the name fred.  Conversely determine if fred is not a member
of the array of names.
The usual way to determine this is to iterate through the entire array
until the item is found (or not found).  Although tedious this is a
quick way to check a small number of items to see if they are part of
the list.
A good technique that I have developed is to use a collection to
achieve this.

============================
Collection Object
A Collection object is an ordered set of items that can be referred to
as a unit.

Remarks
The Collection object provides a convenient way to refer to a related
group of items as a single object. The items, or members, in a
collection need only be related by the fact that they exist in the
collection. Members of a collection don't have to share the same data
type.
A collection can be created the same way other objects are created.
For example:
Dim objCollection As New Collection

Once a collection is created, members can be added using the Add
method and removed using the Remove method. Specific members can be
returned from the collection using the Item method, while the entire
collection can be iterated using the For Each...Next statement.
===================================

When adding items you specify the item value and a key for the item.
I  usually use the same value for the key as for the item.  If the
item is a long string the may be wasteful of memory in which case
alternatives should be sought
Attempting to add an item with the same key as an existing item key
generates a' runtime error 457'.  The error can be trapped  or simply
ignored.
To ignore the error simply wrap the add with an error trap. Eg.

       On Error Resume Next
         objCollection.Add Item, key
      On Error GoTo 0

This will avoid adding the duplicate leaving the original item in the
collection.  If we want to update an existing value (where we don't
know if the item is in the list we can use a similar technique but
this time attempting to remove the item then adding the new item.

     On error resume next
        objCollection.remove "key"
     On error goto 0
     objCollection.Add Item, key

The following example will determine if the item is a member of the
list. If the item exists  the variable temp will contain its value.

       temp = ""
       On Error Resume Next
           temp = objCollection.Item(key)
       On Error GoTo 0

Things to be aware of.
·    Item numbers and item count are dynamic.
·    If an item is removed all subsequent items will shift up and the
item count will reduce by 1.
·    Items can be removed either by key or item number

Since you can have a collection of virtually anything you can address/
store multiple data properties of the item by storing an object in the
collection
Limitation
You can't return an item's index without iterating the list.

METHOD USING A LISTBOX IN A USERFORM
===================================
If programming in a userform we can use the properties of a ListBox
(which can remain hidden) to more easily return the index of an item
if it exists in the list.
Although we can use the same technique as for a collection (above).
The listbox has the advantage that it can easily return the item's
index in the list.
Using a listbox called lbxCTItem

       With lbxCTItem
           .ListIndex = -1    'unselects any selected item
           On Error Resume Next
               lbxCTItem = str
           On Error GoTo 0
           thisItem = .ListIndex
       End With

       'If the item does not exist thisItem retains a value of -1
       'Otherwise it returns the items index number

       If thisItem < 0 Then
               Msgbox "The item does not exist in the list."
       End If

You can store additional data about the item as columns of the
listbox.  By changing the listcolumn number you can even find an item
using that column.  But beware of duplicates.

I Would be interested in hearing comments on this technique.  Maybe
some of the more savvy developers  can comment/test on whether this is
quicker than other methods.  If not I will get around to it one day.
Thill then I suspec that even with several hundred items in a list
most methods will be equally effective in this environment (Document
automation as distinct from number crunching).

Cheers
TonyS.

Guillermo,
Message below in text.

You would do the While loop like this:

Private Function GetNumOfMatchingWords(ArrayToCompare() As String, _
    ReferenceString As String)

    Dim nWord As Long
    Dim WordsFound As Long

    nWord = 1

    nWord = InStr( nWord, ReferenceString, ArrayToCompare(nWord))
    While nWord <> 0
       'aumentamos cómputo de palabras contenidas...
       WordsFound = WordsFound + 1
       nWord = InStr( nWord, ReferenceString, ArrayToCompare(nWord))
    Wend

    GetNumOfMatchingWords = WordsFound
End Function