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                                                                     Exhibit 8.1


                           [Ross & Hardies Letterhead]


SPSS Inc.
233 South Wacker Drive
Chicago, IL 60606

Re: SPSS Inc./ShowCase Corporation Merger

Ladies and Gentlemen:

     You have asked us for our opinion regarding certain U.S. federal income tax
matters in connection with the merger (the "Merger") of SPSS Acquisition Sub
Corp. ("Merger Sub"), a Delaware corporation and a direct wholly-owned
subsidiary of SPSS ("SPSS"), with and into ShowCase Corporation, a Minnesota
corporation ("ShowCase"), to be accomplished pursuant to an Agreement and Plan
of Merger by and among SPSS, Merger Sub and ShowCase, dated as of November 6,
2000 (the "Merger Agreement"). All capitalized terms used but not defined herein
shall have the meaning ascribed to such terms in the Merger Agreement.

     In arriving at the opinions expressed below, we have examined and relied
upon (i) the Merger Agreement, (ii) the Registration Statement on Form S-4, as
filed by SPSS with the Securities and Exchange Commission (the "Commission")
under the Securities Act of 1933, as amended (the "Act"), including the related
Joint Proxy Statement/Prospectus of ShowCase and SPSS (the "Registration
Statement"), (iii) documents delivered in connection with the transactions
contemplated by the Merger Agreement including, without limitation, the
representation letters made by an authorized officer of each of SPSS and
ShowCase, dated as of the date hereof and addressed to us, copies of which are
attached hereto as Exhibits A and B, respectively (the "Representation
Letters"), and (iv) all such documents, instruments and other certificates as we
have deemed appropriate as a basis for the opinions expressed below
(collectively with all documents referred to in this sentence, the "Transaction
Documents").

     In rendering the opinions set forth below, we have relied upon all
statements, facts and representations in the Transaction Documents, and assumed
that all such documents are complete and authentic and have been duly
authorized, executed and delivered. We have further assumed that all statements,
facts and representations made in such documents are true (without regard to any
qualifications stated therein and without undertaking to verify such statements,
facts and representations by independent investigation), that the respective
parties thereto and all
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parties referred to therein will act in all respects to all relevant times in
conformity with the requirements and provisions of such documents, and that none
of the terms and conditions contained therein has been or will be waived or
modified in any respect. We have made such other investigations of fact and law
as we have deemed appropriate as a basis for the opinions expressed below.

     The following opinions are based upon the Internal Revenue Code of 1986, as
amended (the "Code"), applicable Treasury regulations, and rulings and decisions
thereunder, each as in effect on the date hereof, and may be affected by
amendments to the Code or to Treasury regulations thereunder or by subsequent
judicial or administrative interpretation thereof, any of which may have
retroactive effect. We express no opinions other than as to the federal income
tax law of the United States of America. This opinion letter does not address
the various state, local or foreign tax consequences that may result from the
transactions contemplated by the Merger Agreement.

     On the basis of and subject to the foregoing, it is our opinion, as of the
date hereof and under existing law, that

               the consummation of the Merger will be treated for U.S. federal
               income tax purposes as a reorganization within the meaning of
               Section 368(a) of the Code,

               SS, Merger Sub and ShowCase will each be a party to the
               reorganization within the meaning of Section 368(b) of the Code.

     We hereby consent to the filing of this opinion letter with the Commission
as an exhibit to the Registration Statement. We also consent to the reference to
our firm under the heading "The Merger - Material Federal Income Tax
Consequences of the Merger" in the Joint Proxy Statement/Prospectus. In giving
this consent, we do not admit that we are within the category of persons whose
consent is required under Section 7 of the Act or the General Rules and
Regulations of the Commission.

     This opinion letter is furnished to you solely for your benefit in
connection with the preparation of the Registration Statement and is not to be
used, circulated, quoted or otherwise referred to for any other purpose or
relied upon by any other person without our express written permission.

     We expressly disclaim any obligation or undertaking to update or modify
this opinion letter as a consequence of any future changes in applicable laws or
Treasury regulations or the facts bearing upon this opinion letter, any of which
could affect our conclusions.


                            Very respectfully yours,


                                 ROSS & HARDIES

                                 /s/ ROSS & HARDIES
                                 -----------------------
                                       By: A partner