All posts by Li Cai

Remembering Professor R. Darrell Bock

It is with great sadness that we acknowledge the passing of R. Darrell Bock. Among his many incredible contributions to science and psychometrics, Professor Bock was also co-founder and long-time President of SSI. The company would never have existed without Professor Bock and the history of scientific software would have been drastically different without his contributions. The staff at SSI express their heartfelt condolences to Professor Bock’s family and friends. For more information on his amazing life and contributions, you can find his obituary (penned by Robert Gibbons and Tania Bock) here, as well as an article remembering his life and contributions in UChicago News.

VPG Releases New Version of IRTPRO

SSI’s parent company, Vector Psychometric Group, LLC, released IRTPRO 5.

IRTPRO™ is an application for comprehensive item calibration and test scoring using IRT models. IRTPRO™ was first released in 2010.  The current version of IRTPRO™ is Version 5.0 (released date: June 17, 2020).

As of June 17, 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 to purchase a subscription or to manage your IRTPRO license. 

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

All questions about IRTPRO™ sales should be directed to  Feel free to continue to write to 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

HLM 8 Now Available on SSI Live™

HLM transitions to the SSI Live™ software-as-subscription service.

We are pleased to announce that after several years of development, SSI is offering a brand new HLM software licensing, delivery, and support model. We call it SSI Live™. HLM is the first program to transition to the new model. SSI Live™ is a subscription, similar to the popular rental licenses that we used to market, but with many more features.With your renewable SSI Live subscription, you are entitled to many more benefits in comparison to previous licensing models.  Click here to view a side-by-side comparison chart of features.

The following subscriptions are available:

Standard subscriptions for 3, 6 or 12 months:

This subscription is a standard non-commercial, academic, and educational copy of HLM. While your renewable subscription is active, key benefits include: 

  • download and simultaneously install state-of-the-art HLM program on two (2) computers  
  • additional installs can be purchased and added at any time at discount
  • free access to all upgrades or updates 
  • free software support and technical support 
  • additional discounts on subscriptions of other SSI or VPG programs 

Basic subscriptions for 3 or 12 months:

This subscription is a basic non-commercial, academic, and educational copy of HLM. While your renewable subscription is active, you are entitled to the following benefits: 

  • download and install state-of-the-art HLM program on one computer  
  • free access to all upgrades or updates 

Trial version:

  • A fully-functioning 14-day trial subscription is available for a potential user’s evaluation purposes.  
  • For educators who plan to use HLM in instruction, your students may be eligible access the full version of HLM for free during your course or workshop. 

To acquire a subscription or to find out more about SSI Live™, please visit For help on registering, see our Quickstart guide. For more information on adding additional users to a license, see our FAQ on this topic

Current users with perpetual licenses will be able to continue to use the program according to the terms of the end user license agreement. Over time, this transition will impact on the technical support we offer our users who hold older perpetual licenses. 

HLM 8 users:

Please refer to an email recently sent from sharing more details on how to activate your subscription account. 

HLM 6 users:

As you are aware, the final release of HLM 6 occurred in 2009, which is more than a decade ago.  The current release of HLM is Version 8. Software products built on older operating systems and third-party components become increasingly more difficult to maintain and support, and as a consequence, also less secure.  Due to such concerns, we will phase out support for HLM 6 at the end of this calendar year (12/31/2020).  Because you have a perpetual license, you certainly may continue to use the program according to the terms of the end user license agreement beyond this year.  However, as of January 1, 2021, our HLM Support Desk will not respond to HLM 6 inquiries. 

SSI Acquired by Vector Psychometric Group

Scientific Software International Inc. is acquired by Vector Psychometric Group.

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.