The quality of the software and the design of the system in which it is used. There are several factors that can affect the reliability of software, including:
- Complexity: The more complex the software, the more difficult it is to test and debug, which can lead to reliability issues. 
- Quality of the code: Poorly written or poorly organized code is more likely to contain bugs and defects, which can compromise the reliability of the software. 
- Testing: Adequate testing is essential for identifying and fixing bugs and defects before the software is released. 
- User error: The reliability of the software can be affected if it is used in a way that it was not designed for or if it is misused by the user. 
- Environmental factors: The reliability of the software can be affected by the operating system, hardware, and other environmental factors. 
- Maintenance: Proper maintenance and updates can help to ensure the continued reliability of the software.