Software development requires multi-stage processes in order to organise the software development effort. Each software development project should implement a development process that is appropriate to the project setting. Since business needs and technologies are subject to change, software process improvement (SPI) actions are required so as to harmonise the process with the emerging business and technology needs. SPI frameworks such as CMMI and ISO-15504 have been designed to support SPI efforts. While SPI frameworks have been shown to be beneficial for large organisations, they are not widely implemented in small to medium sized enterprises (SMEs), where evidence of the positive influence of SPI on business success is weak. This paper is proposing that a new measure, the SPI Key Performance Indicator (SPI-KPI), is developed to investigate the association between SPI and business success in SMEs, with the objective of demonstrating that SPI is positively associated with business success in SMEs.