A statistical approach has been demonstrated in the present paper to interpret in-situ plate load test data conducted for the assessment the deformability characteristics of rock mass. Such statistical analysis provides the values of deformation modulus with desired level of confidence. These values may be used for risk analysis by conducting parametric studies using numerical modeling tools. Rock mass deformation characteristics have been discussed in this paper based on the 6 plate load tests conducted on granite rock mass at Kirthai hydroelectric project (Stage-II), J&K and 16 plate load tests conducted on phyllite rock mass at Amochu hydroelectric project, Bhutan. Study of this limited data indicated that the total deformation occurred during the uni-axial loading is lognormally distributed. Using the proposed distribution parameters, in-situ rock mass deformation is estimated with 95%, mean, and 5% confidence levels and corresponding deformation modulus is also estimated. This statistical approach has been employed to estimate the upper and lower bound on the values of deformation modulus which are compared with the deterministic estimates.