<PAGE>

                                                                   EXHIBIT 10.51

                    AMENDED AND RESTATED CONSULTING AGREEMENT

      This AMENDED AND RESTATED CONSULTING AGREEMENT (the "Agreement"), dated as
of January 1, 2005 (the "Effective Date"), is by and between SPSS Inc., a
Delaware corporation ("SPSS"), and Norman H. Nie Consulting L.L.C., an Illinois
Limited Liability Company, (the "Consultant").

                              W I T N E S S E T H:

      WHEREAS, SPSS and Consultant previously entered in a consulting agreement
dated June 1, 2003 pursuant to which the Consultant performed various duties for
SPSS (hereinafter "Prior Agreement"); and

      WHEREAS, SPSS and Consultant now desire to amend and restate the terms and
conditions of the Prior Agreement by entering into this Agreement, the terms of
which shall replace the terms of the Prior Agreement and govern the relationship
between the parties hereto regarding the subject matter set forth herein as of
the Effective Date.

      NOW, THEREFORE, in consideration of the premises, the representations,
warranties, covenants and agreements contained herein, and other good and
valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties hereto do hereby agree as follows:

                                    ARTICLE 1

                              CONSULTING AGREEMENT

      1.1 Consultancy. Subject to the terms and conditions of this Agreement,
SPSS agrees to retain Consultant, and Consultant agrees to provide consulting
services as documented in this Agreement.

      1.2 Consulting Services. Consultant shall perform the duties and have the
responsibilities set forth in Schedule 1.2 attached hereto and incorporated
herein by reference. SPSS and Consultant may amend Schedule 1.2 in accordance
with the terms of Section 4.9 hereof.

<PAGE>

      1.3 Compensation. Consultant shall be paid a fee of $15,000 per month for
the consulting services set forth in Section 1.2 above. Consultant shall be paid
on the last day of each calendar month for the just completed month. If the
Agreement is terminated in the middle of the month, Consultant shall be paid a
pro-rata portion of the fee owed to Consultant for such month.

      1.4 Term and Termination. This Agreement shall continue until one of the
parties sends the other party fifteen (15) days notice that it wishes to
terminate this Agreement. The parties agree to discuss on a quarterly basis
whether the services being provided by Consultant to SPSS are still needed.

      1.5 Expenses. In addition to the compensation payable under Section 1.3
hereof, with the advance approval of the Chief Financial Officer, Consultant
shall be entitled to reimbursement for his reasonable out-of-pocket expenses
incurred in performing requested services under this Agreement.

                                    ARTICLE 2

                                  GENERAL TERMS

      2.1 Relationship Between the Parties. The relationship of Consultant
acting in his capacity as a consultant pursuant to Article I hereof, to SPSS
hereunder is, and shall remain, that of an independent contractor. Nothing in
this Agreement shall be deemed to constitute an employee/employer, partnership
or fiduciary relationship between the parties. Except as specifically provided
herein, nothing in this Agreement shall be deemed to constitute either party as
the agent of the other, nor shall either party have the right to bind the other
party or make any promises or representations on behalf of the other.

      2.2 Confidential Information. The Consultant shall not at any time during
or for a period of five after the expiration or termination of this Agreement,
except pursuant to an order of any court of competent jurisdiction,
administrative agency or other governmental entity having authority to so
require, and except for the purposes of any tax return and/or report required to
be made to any taxing authority, directly or indirectly, divulge, furnish, or
cause to be

<PAGE>

divulged or furnished to any individual or entity, other than SPSS or any
employee of SPSS, or make any use for his own benefit, or for the benefit of any
person, firm, corporation or other entity, other than SPSS or an affiliate
thereof, any secret or confidential information of SPSS, including but not
limited to, the names of customers, customer information, financial information,
technical information, supplier information, details or information concerning
contracts, trade secrets, marketing information, or any other data, information
or proprietary information of or relating to the Business, SPSS or any affiliate
thereof, or their respective products or services, to the extent not generally
known within the trade or not a matter of public knowledge and which was
acquired by the Consultant during his employment with SPSS or obtained in
connection with his duties hereunder during the term of this Agreement.

      2.3 Non-Competition. The Consultant hereby covenants and agrees that, for
the period of the consultancy and for a period of one year thereafter, the
Consultant shall not (i) directly or indirectly (whether through a partnership
of which the Consultant is a partner or through any other individual or entity
in which the Consultant has any interest, legal or equitable), engage in any
business competitive with the business of SPSS or its subsidiaries and
affiliates, (ii) directly or indirectly (whether through a partnership of which
the Consultant is a partner or through any other individual or entity in which
the Consultant has any interest, legal or equitable), solicit or otherwise
engage with (except pursuant to the Consultant's consultancy with SPSS) any
customers or clients of SPSS or its subsidiaries or affiliates, in any
transactions which are in direct competition with the business of SPSS or its
subsidiaries or affiliates, or (iii) directly or indirectly (whether through a
partnership of which the Consultant is a partner or through any other individual
or entity in which the Consultant has any interest, legal or equitable), assist
any person in the development, programming, servicing, maintenance, manufacture,
sale, licensing, distribution or marketing (including, without limitation,
giving away software) of software and related products in competition with SPSS
or any of its affiliates' products, in each case in the United States of America
or any country where SPSS or its subsidiaries or affiliates are doing

<PAGE>

business, excluding passive investment interests of less than two percent (2%)
in corporations whose stock is registered under the Securities Exchange Act of
1934, as amended.

                                    ARTICLE 3

                                   INVENTIONS

      3.1 Definition of Inventions. "Inventions" mean all systems, programs,
algorithms, procedures, techniques, manuals, data bases, plans, lists,
inventions, copyrights, patents, trademarks, discoveries, innovations, concepts,
ideas and software (including without limitation source and object code and
design and user documentation) conceived, compiled or developed by Consultant in
the course of the Consultant's engagement by SPSS.

      3.2 Consultant shall promptly disclose to SPSS the products of the work
performed hereunder, and those products will be deemed to be a "work made for
hire"; and SPSS shall be considered to be the person for whom the work was
prepared under the copyright laws of the United States.

      3.3 Assignment of Inventions. For value received, Consultant hereby
assigns to SPSS Consultant's sole and exclusive right, title and interest in and
to (a) any Invention developed by the Consultant for SPSS, and (b) any Invention
developed, in whole or in part, using Confidential Information of SPSS.

      3.4 Further Assurances. Consultant agrees to execute and deliver any
instruments or documents and to do all other things (including, without
limitation, the giving of testimony) reasonably requested by SPSS (both during
and after Consultant's engagement by SPSS) in order to vest more fully in SPSS
all ownership rights in the Inventions covered by Section 3.2 hereof (including,
without limitation, securing patent and copyright registrations for such
Inventions). SPSS will reimburse Consultant for reasonable out-of-pocket
expenses incurred by Consultant to comply with the foregoing.

      3.5 Notice. During the term of Consultant's engagement and for a period of
12 months thereafter, Consultant will promptly notify SPSS of each Invention and
include a reasonably detailed description thereof in order that a determination
can be made as to whether the Invention in question is subject to this
Agreement.

      3.6 Copyrights and Patents. In addition to the assignment of Inventions
made above, Consultant also assigns to SPSS the entire right, title and interest
to any copyrights and any work which may not be deemed "work made for hire"
under the copyright laws, but which is produced

<PAGE>

by Consultant in accordance with the terms of this Agreement. Consultant further
agrees to execute any documents which may be necessary or appropriate to allow
SPSS to perfect its interest in the copyright for such works. Consultant shall
not be entitled to any additional payment or compensation for assisting and
cooperating with SPSS in obtaining these copyrights, except for reimbursement of
reasonable expenses incurred by Consultant in performing his obligations under
the previous sentence.

      3.7 SPSS shall have title to any Inventions which are made during the
course of work under this Agreement as well as any patents thereon in all
countries. If Consultant hires employees or subcontractors to perform his
obligations hereunder, if Consultant agrees to enter into agreements with such
employees or subcontractors to protect the Confidential Information and
Inventions for the benefit of SPSS to the same extent provided for hereunder,
and to submit a duplicate original of all such agreements to SPSS prior to the
rendering of any services by any employee or subcontractor under this Agreement.
Consultant agrees to give SPSS or any party designated by SPSS all assistance
reasonably required to perfect the rights described above.

                                    ARTICLE 4

                                  MISCELLANEOUS

      4.1 Equitable Relief. The Consultant understands that a breach by it of
any provision of this Agreement may cause substantial injury to SPSS which may
be irreparable and/or in amounts difficult or impossible to ascertain, and that
in the event the Consultant breaches any provision of this Agreement, SPSS shall
have, in addition to all other remedies available in the event of a breach of
this Agreement, the right to injunctive or other equitable relief. Further, the
Consultant acknowledges and agrees that the restrictions and commitments set
forth in this Agreement are necessary to protect SPSS's legitimate interests and
are reasonable in scope, area and time, and that if, despite this acknowledgment
and agreement, at the time of the enforcement of any provision of this Agreement
a court of competent jurisdiction shall hold that the period or scope of such
provision is unreasonable under the circumstances then existing, the maximum
reasonable period or scope under such circumstances shall be substituted for the
period or scope stated in such provision.

<PAGE>

      4.2 Notices. Any notice, request, instruction or other document to be
given hereunder by one party hereto to the other party hereto shall be in
writing and delivered personally (effective upon delivery), sent by overnight
courier freight prepaid (effective one day after delivery to such courier during
its regular business hours), sent by registered or certified mail, postage
prepaid (effective 5 days after deposit in the U.S. Mail) or sent by facsimile
transmission (effective upon confirmation of receipt),

                if to Consultant to:

                                            Norman H. Nie Consulting
                                            SIQSS - Stanford
                                            417 Galvez Mall
                                            Encina West, Room 104
                                            Stanford, CA 94035-6074
                                            Attention:  Norman H. Nie

                if to SPSS to:              SPSS Inc.
                                            233. South Wacker Drive
                                            11th Floor
                                            Chicago, IL  60606
                                            Attention: Chief Financial Officer
                                            Facsimile number:  (312) 329-3560

or to such other address as shall be provided to the other persons named herein
pursuant to notice given pursuant to the provisions of this Section 4.2.

         4.3 Arbitration. Any dispute with regard to the terms of this Agreement
shall be settled by arbitration in the City of Chicago, Illinois by three
arbitrators, one of whom shall be appointed by Consultant, one of whom shall be
appointed by SPSS and the third of whom shall be appointed by the first two
arbitrators. If either party fails to appoint an arbitrator within 20 days of a
request in writing by the other party to do so, or if the first two arbitrators
cannot agree on the appointment of a third arbitrator within 20 days of their
designation, then such arbitrator shall be appointed by the Chief Judge of the
United States District Court for the Northern District of Illinois. Except as to
the selection of arbitrators which shall be as set forth above, the

<PAGE>

arbitration shall be conducted promptly and expeditiously in accordance with the
commercial arbitration rules of the American Arbitration Association so as to
enable the arbitrators to render an award within 90 days of the commencement of
the arbitration proceedings. Judgment upon the award rendered by the arbitrators
may be entered in any court having jurisdiction thereof. Consultant, on the one
hand, and SPSS, on the other hand, shall each bear one-half of the expenses of
the arbitration; except that, in the case where the parties are unable to agree
on a single arbitrator, each party shall bear the expenses of the arbitrator it
selects.

      4.4 Assignment. Neither this Agreement nor any of the rights, interests or
obligations hereunder shall be assigned by either of the parties, except by
operation of law and except that SPSS may assign its rights and obligations
under this Agreement to any affiliate of SPSS. If such assignment shall be made
by SPSS, such affiliate shall be entitled to all of the rights and shall assume
all of the obligations of SPSS hereunder, provided, that SPSS shall remain
liable for the performance of such affiliate's obligations hereunder.

      4.5 Effect and Benefit. This Agreement shall be binding upon and inure to
the benefit of the heirs and personal representatives of the Consultant and to
the successors and assigns of SPSS.

      4.6 Governing Law. The validity, interpretation and performance of this
Agreement shall be governed and construed in all respects in accordance with the
internal substantive laws of the State of Illinois, without regard to its
conflicts of law principles.

      4.7 Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall constitute an original but all of which
together shall constitute one and the same agreement.

      4.8 Severability. The provisions of this Agreement shall be severable, and
the invalidity of any one or more of such provisions shall not affect the
validity of any of the other provisions hereof.

<PAGE>

      4.9 Amendment and Modification. No amendment, modification or alteration,
nor any waiver, of the terms and conditions of this Agreement shall be binding
unless the same shall be in writing and duly executed by both SPSS and
Consultant.

      4.10 Waiver of Breach. The waiver by one party of a breach of any
provision of this Agreement by the other party shall not operate or be construed
as a waiver of any subsequent breach of the same or any other provision of this
Agreement by that party.

      4.11 Expenses. Except as otherwise provided, the Consultant and SPSS shall
each pay all costs and expenses incurred by him or it or on his or its behalf in
connection with this Agreement and the transactions contemplated hereby,
including, without limiting the generality of the foregoing, fees and expenses
of its own financial consultants, accountants and counsel.

      4.12 Prior Agreement. SPSS and Consultant hereby agree that the terms of
this Agreement shall supersede and replace the terms of the Prior Agreement and,
upon execution of this Agreement, the terms of the Prior Agreement shall no
longer be in effect.

      4.13 Acknowledgement. SPSS acknowledges that Consultant is a co-founder of
Knowledge Networks and currently owns approximately 2.1% of such company.

<PAGE>

            IN WITNESS WHEREOF, each of the parties has caused this Consulting
Agreement to be duly executed and delivered as of the day and year first above
written.

                                    SPSS INC.

                                    By: /s/ Raymond H. Panza
                                        ----------------------------------------
                                    Name: Raymond H. Panza
                                    Its:  Executive Vice President, Corporate
                                          Operations, Chief Financial Officer
                                          and Secretary

                                    NORMAN H. NIE CONSULTING, L.L.C.

                                    By: /s/ Norman H. Nie
                                        ----------------------------------------
                                    Name: Norman H. Nie
                                    Its:  President

<PAGE>

                                  Schedule 1.2

DUTIES AND RESPONSIBILITIES:

Consultant's duties and responsibilities will consist of:

      1)    Assisting with product management responsibilities for the SPSS
            family of products;

      2)    Assisting the technology group with development of SORs and
            prioritization of new products and features in the areas of
            statistics, survey research and text mining.

In the role set forth above, Consultant will have no budgetary authority.
Consultant will advise and can provide recommendations into where the money is
being spent. The final project and investment decisions are the responsibility
of the SPSS employee in charge of the budget.