IMS Health Incorporated
                                200 Nyala Farms

                              Westport, CT 06880

                                                                 June 29, 1998

Nancy Henry, Esq.
The Dun & Bradstreet Corporation
One Diamond Hill Road
Murray Hill, NJ 07974

Earl Doppelt, Esq.
ACNielsen Corporation
177 Broad Street
Stamford, CT  06901

Dear Ms. Henry and Mr. Doppelt:

            Reference is made to the Distribution Agreement (the "1996
Distribution Agreement"), dated as of October 28, 1996, among Cognizant
Corporation ("Cognizant"), The Dun & Bradstreet Corporation ("D&B") and
ACNielsen Corporation ("ACNielsen"). Cognizant has announced its intention to
separate into two separate companies through a distribution (the "IMS HEALTH
Distribution") to its stockholders of all of the shares of common stock of its
subsidiary IMS Health Incorporated ("IMS HEALTH"). In Section 8.9(c) of the 1996
Distribution Agreement, Cognizant agreed not to make a distribution such as the
IMS HEALTH Distribution unless it caused the distributed entity to undertake to
both D&B and ACNielsen to be jointly and severally liable for all Cognizant
Liabilities (as defined in the 1996 Distribution Agreement). Therefore, in
accordance with Section 8.9(c) of the 1996 Distribution Agreement and intending
to be legally bound hereby, from and after the effective time of the IMS HEALTH
Distribution, IMS HEALTH undertakes to each of D&B and ACNielsen to be jointly
and severally liable with Cognizant for all Cognizant Liabilities under the 1996
Distribution Agreement.

                                          Very truly yours,

                                          IMS HEALTH INCORPORATED

                                          By:   /s/ Kenneth S. Siegel
                                                --------------------------------
                                                Name:  Kenneth S. Siegel
                                                Title: Senior Vice President,
                                                            General Counsel and
                                                            Secretary