We are happy to announce that in early 2020, Scientific Software International Inc., became a wholly owned subsidiary of Vector Psychometric Group, LLC, a North Carolina-based software and consulting company. We believe this exciting new development in SSI’s corporate history will reinvigorate the SSI products that have for so long formed the backbone of research in a variety of fields.

An important part of this evolution of SSI is the gradual roll-out of a new software delivery and support model for all our programs. We call it SSI Live™. 

HLM is the first program to transition. After purchasing an SSI Live™ Standard subscription, you will be entitled to:

1) download multiple installs (not possible previously unless with purchasing a special license),

2) free access to all upgrades or updates during the period where the subscription is active,

3) free software support, and

4) discounts on subscriptions to other SSI programs.  

Additional educator benefits are included. Visit SSI Live™ for more information.


Software products

The specialized statistical software published and distributed by SSI are used for data analysis in fields such as

  • Statistics (including survey research and informatics)
  • Behavioral and social sciences (such as psychology, sociology, psychiatry, criminal science, family studies, political science, developmental research, anthropology, or social work)
  • Medical research (including nursing, pharmacy, epidemiology, gerontology, kinesiology, sport science, and other fields)
  • Education (in administration, policy studies, test analysis, counseling, and more)
  • Business research (marketing, management, economics, organization)
  • Environmental science (including resource administration and longitudinal research)
  • Other diverse research areas, such as language studies, engineering, law, chemistry, or urban planning and include:


  • HLM 
    HLM provides statistical tools for comprehensive hierarchical data modeling. A recent addition to this popular program is the option to estimate an HLM from incomplete data, using a completely automated approach that generates and analyses multiply imputed data sets from incomplete data.



    IRTPRO™ is an application for comprehensive item calibration and test scoring using IRT models. IRT models for which item calibration and scoring are implemented in IRTPRO™ are based on unidimensional and multidimensional [confirmatory factor analysis (CFA) or exploratory factor analysis (EFA)] versions of the widely used response functions such as 1PL, 2PL, 3PL, graded, generalized partial credit, and nominal.  More information about IRTPRO™ can be found at irtpro.com. IRTPRO™ was first released in 2010.  The current version of IRTPRO™ is Version 5.0 (released date: June 19, 2020).

    As of June 19, 2020, IRTPRO™ is no longer marketed by SSI.  Instead, SSI’s parent company, Vector Psychometric Group, LLC, is exclusively marketing IRTPRO™ under a subscription-based software delivery and support model.  Please visit the updated online store at https://store.vpgcentral.com/ to purchase a subscription or to manage your IRTPRO license. 

    All questions about IRTPRO™ sales should be directed to This email address is being protected from spambots. You need JavaScript enabled to view it..  Feel free to continue to write to This email address is being protected from spambots. You need JavaScript enabled to view it. if you have any technical questions about IRTPRO™ (Version 2 – Version 4). IRTPRO™ 5+ users should write to the new, dedicated IRTPRO™ customer support desk at This email address is being protected from spambots. You need JavaScript enabled to view it..


  •  LISREL 
    LISREL provides tools for structural equation modeling, data manipulations and basic statistical analyses, hierarchical and non-linear modeling, generalized linear modeling, and generalized linear modeling for multilevel data.


    BILOG-MG is an extension of BILOG that is designed for the efficient analysis of binary items, including multiple-choice or short-answer items scored right, wrong, omitted, or not-presented. BILOG-MG is capable of large-scale production applications with unlimited numbers of items or respondents.


    PARSCALE implements Item Response Theory (IRT) measurement methods for binary, multiple-category and rating-scale items. The flexibility and the wealth of information provided by this program have kept it in regular use by researchers around the world.


  • SuperMix 
    SuperMix is an application for comprehensive mixed-effect modeling and combines the functionality of four mixed-effects programs, MIXREG, MIXOR, MIXNO, and MIXPREG, developed by Donald Hedeker and Robert Gibbons into a single application to provide estimates for mixed-effects regression models.


  • Auxal
    Auxal performs both structural and nonstructural auxological analysis of human growth in standing height or recumbent length.