The traditional analysis of a slope stability proceeds by first testing the soil at a limited number of locations to assess its properties (for example by CPT). Then simplified slope stability analysis methods are commonly employed to design the slope. In reality, limited site investigation tests cannot be representative of the whole slope due to spatial variability of soil properties. Inadequate site investigation may lead to under-design or over-design of slopes. It is therefore necessary, to carry out the site investigation tests which can provide most adequate information for use in design. As such, this paper proposed a framework to quantify the probability of under-design or overdesign for different site investigation strategies based on Monte Carlo simulation. A slope is used to assess the reliability of various site investigation scopes. It is observed, that the probability of under-design or over-design decreases as the scope of the site investigation increases.