SIXTH SUPPLEMENTAL INDENTURE

Exhibit 4.1(a)

EXECUTION VERSION

SIXTH SUPPLEMENTAL INDENTURE

Sixth Supplemental Indenture (this “Supplemental Indenture”), dated as of April 16, 2008, among Nielsen Finance LLC, a Delaware limited liability company, and Nielsen Finance Co., a Delaware corporation (the “Issuers”), the Guarantors (as defined in the Indenture referred to below) and Law Debenture Trust Company of New York, as trustee (the “Trustee”).

WITNESSETH

WHEREAS, the Issuers and the Guarantors have heretofore executed and delivered to the Trustee an indenture dated as of August 9, 2006, as supplemented by the First Supplemental Indenture, dated October 16, 2006, Second Supplemental Indenture, dated August 15, 2007, Third Supplemental Indenture, dated August 15, 2007, Fourth Supplemental Indenture, dated November 28, 2007 and Fifth Supplemental Indenture, dated April 9, 2008 (the “Indenture”), providing for the issuance of $650.0 million aggregate principal amount of 10.0% Senior Notes due 2014 (the “Initial Notes”);

WHEREAS, pursuant to Section 2.01 of the Indenture, the Issuers may create and issue, from time to time without notice to or consent of any holder of the Initial Notes, additional notes that are subject to the provisions of the Indenture upon written order of the Issuers to the Trustee in the form of an Authentication Order specifying the amount and series of such notes to be authenticated and the date on which the notes are to be authenticated by the Trustee; provided that the Issuers’ ability to issue such additional notes shall be subject to the Issuers’ compliance with Section 4.09 of the Indenture;

WHEREAS, the Issuers are issuing an additional $220,000,000 aggregate principal amount of their 10.0% Senior Notes due 2014 (the “Additional Notes”) with the same terms as the Initial Notes;

WHEREAS, pursuant to Section 9.01 of the Indenture, the Issuers, the Guarantors and the Trustee are authorized to execute and deliver this Supplemental Indenture without the consent of the holders and the conditions set forth in the Indenture for the execution and delivery of this Supplemental Indenture have been complied with; and

WHEREAS, all other actions necessary to make this Supplemental Indenture a legal, valid and binding agreement of the Issuers, the Guarantors and the Trustee, in accordance with its terms, and a supplement to, the Indenture, have been performed;

NOW THEREFORE, in consideration of the foregoing and for other good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, the parties mutually covenant and agree for the equal and ratable benefit of the Holders of the Notes as follows:

(1) Definitions. Capitalized terms used herein without definition shall have the meanings assigned to them in the Indenture.

(2) Provisions of the Indenture. The provisions of the Indenture shall apply to the Additional Notes and any certificated Additional Notes in definitive form issued in exchange therefore in the exact same manner as they apply to the Initial Notes and any certificated Initial Notes in definitive form issued in exchange therefore.

(3) Effect of Supplemental Indenture.

(a) This Supplemental Indenture is a supplemental indenture within the meaning of Section 9.01 of the Indenture, and the Indenture shall be read together with this Supplemental Indenture and shall have the same effect over the Additional Notes, in the same manner as if the provisions of the Indenture and this Supplemental Indenture were contained in the same instrument.

(b) In all other respects, the Indenture is ratified and confirmed by the parties as supplemented by the terms of this Supplemental Indenture.

(4) Note Forms. The Additional Notes issued by the Issuers pursuant to this Supplemental Indenture shall be substantially in the form of Exhibit A-1 to the Indenture, except that, for purposes of the Additional Notes issued by the Issuers pursuant to this Supplemental Indenture, the interest payment in Section 1 shall run from April 16, 2008, and the first Interest Payment Date shall be August 1, 2008.

The Notes may have notations, legends or endorsements required by law, stock exchange rule or usage in addition to those set forth in Exhibit A-1 to the Indenture.

(5) Governing Law. THIS SUPPLEMENTAL INDENTURE WILL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

(6) Counterparts. The parties may sign any number of copies of this Supplemental Indenture. Each signed copy shall be an original, but all of them together represent the same agreement.

(7) Effect of Headings. The Section headings herein are for convenience only and shall not affect the construction hereof.

(8) The Trustee. The Trustee shall not be responsible in any manner whatsoever for or in respect of the validity or sufficiency of this Supplemental Indenture or for or in respect of the recitals contained herein, all of which recitals are made solely by the Issuers.

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(9) Severability Clause. In case any provision in this Supplemental Indenture or in the Additional Notes shall be invalid, illegal or unenforceable, the validity, legality and enforceability of the remaining provisions shall not in any way be affected or impaired thereby.

(10) Successors. All agreements of the Issuers and the Guarantors in this Supplemental Indenture shall bind their successors. All agreements of the Trustee in this Supplemental Indenture shall bind its successors.

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IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to be duly executed, all as of the date first above written.