Applying the SP1 update to Vista does nothing to improve its application compatibility, with almost 20 percent of software still unable to run, a study has found.

Recently-founded application testing company AppDNA ran a 'before' and 'after' on Vista using SP1 (service pack 1) using its AppTitude system, finding that it made no difference whatsoever to its ability to run a test suite of corporate 500 applications, 18 percent of which failed to work without help.

As bad as this sounds, the company rated this as good news because service packs normally introduce as many problems as they solve. That SP1 left compatibility performance unchanged was in stark contrast to the experience with SP2 for Windows XP, which unexpectedly broke many applications that had worked before.

"Organisations have been waiting to see SP1. But people have been wary after SP2 [for XP]," said AppDNA's CTO, Paul Schnell. "The updates have been incremental and had no impact on compatibility."

The AppTitude testing software used an algorithmic technique to look at applications for behaviour known to break Vista. The company also ran usability tests by loading software and digging around to see whether there were issues, Schnell said.

Customers running the tests on their own applications using AppTitude would receive an indication of the severity of compatibility issues using a red, amber and green light system, giving them the information needed to plan migration. The software also provided a report on possible fixes.

According to its official history, AppDNA is in its first month of independent operation, having been spun out of application migration specialist Camwood, which remains a channel partner. The company won't be keen to annoy Microsoft too much - its early business has it working closely with the latter on migration projects.

Equally, Microsoft has long given up worrying about the Vista's difficult image. The news that it might still have issues with one in five applications won't come as a surprise.

Redmond has announced its own application migration tools for Vista and prepared for the long haul. The company has pursued a policy of open access to the SP1 refresh, making as few claims as possible regarding the specific issue of compatibility.