Exhibit 99 (b)


                         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