The stellar mass function is one of the fundamental distributions of stellar astrophysics .
Its form at masses similar to the Sun was found by Salpeter ( 1955 ) to be a power-law m ^ { - \alpha } with a slope of \alpha = 1.35 .
Since then the mass function in the field , in stellar clusters and in other galaxies has been studied to identify variation due to environment and mass range .
Here we use results from previous papers in the SIPS series to constrain the mass function of low mass stars ( 0.075M _ { \odot } < m < 0.2 _ { \odot } ) .
We use simulations of the low mass local stellar population based on those in Deacon & Hambly ( 2006 ) to model the results of the SIPS-II survey ( Deacon & Hambly , 2007 ) .
We then vary the input parameters of these simulations ( the exponent of the mass function \alpha and a stellar birthrate parameter \beta ) and compare the simulated survey results with those from the actual survey .
After a correction for binarity and taking into account potential errors in our model we find that \alpha = -0.62 \pm 0.26 for the quoted mass range .