## 3 Results and Discussion
### Best Practices Evaluation for Sustainability  
We collected data on a set of best practices for sustainability. In Figure 2, we compare the percentage of application of these practices, along with the percentage of agreement on suitability for sustainability. The figures reveal that \"Reuse of code\" (92.55%), \"Reuse of software requirements and specifications\" (72.34%), and \"Testing and continuous integration\" (67.02%) were the most common practices that respondents applied. Moreover, reuse practices were one of the most convincing alternatives that produce the greatest impact on sustainability, with over 80% agreement. This could be explained by the availability of know-how in companies, as well as by the quick access to it. This also leads to a speeding-up of the software development processes, making it more possible to save company resources and costs. \"Testing and continuous integration\" produces a conviction about sustainability that is slightly different, with between 60% and 80% agreement. This difference could be related to the inherent need for software development iterations, and to the uncertain results produced in each iteration. Apart from reuse practices and continuous integration, \"Standardization of practices, tools, technical solutions, etc.\", \"General sustainability policies\", \"Specific sustainability requirements\" and \"Lean and agile methodologies\" were perceived as the most effective ways of reaching sustainability. There was a consensus about this among respondents, with over 80% agreement. In addition, these practices provide clear steps on how to proceed when developing software, increasing the sustainability expertise in companies. \"Standardization of practices, tools, technical solutions, etc.\" (56.38%) and \"Lean and agile methodologies\" (57.45%) also had an acceptable percentage of usage. On the other hand, \"General sustainability policies\" (25.53%) and \"Specific sustainability requirements\" (14.89%) are not commonly applied at work. This low percentage can be explained by the lack of knowledge and literature/sources, which leads to little time being devoted to sustainability. Consequently, there seems to be a need to promote these practices so that professionals apply them with the same frequency as other practices. \"Cloud computing and virtualization services\" (59.57%) and \"Usability and user experience requirements or principles in the development of products\" (56.38%) are widespread practices in software development companies. Moreover, there is a high level of agreement (between 60% and 80%) among practitioners that these practices help towards sustainability. The existence of certifications specifically designed for data centers, such as the Certified Energy Efficient Data Center Award (CEEDA), may explain the trust in one of these practices. The existence of companies certified in sustainability increases confidence on the part of customers when outsourcing of services occurs. On the other hand, usability and user experience requirements are widely studied, and there are many catalogs available in the literature. Other practices reached a good level of conviction among the practitioners (also between 60% and 80%). They were \"Use of \"green\" datacenter infrastructures (that use renewable energy)\", \"Taking sustainability into consideration when software systems and services are purchased or outsourced\", \"Accessibility requirements or principles in the development of products\", \"Internationalization requirements or principles in the development of products\", and \"Certification in any sustainability regulation (for example, ISO 14001, EMAS...)\". However, not all of these are widely used. The most commonly applied at work are \"Accessibility requirements or principles in the development of products\" (40.43%) and \"Internationalization requirements or principles in the development of products\" (40.43%).