  The Multi Aperture Scintillation Sensor (MASS) and the
Generalized-Scintillation Detection and Ranging (Generalized SCIDAR) are two
instruments conceived to measure the optical turbulence (OT) vertical
distribution on the whole troposphere and low stratosphere (~ 20 km) widely
used in the astronomical context. In this paper we perform a detailed
analysis/comparison of measurements provided by the two instruments and taken
during the extended site testing campaign carried out on 2007 at Cerro Paranal
and promoted by the European Southern Observatory (ESO). The main and final
goal of the study is to provide a detailed estimation of the measurements
reliability i.e dispersion of turbulence measurements done by the two
instruments at different heights above the ground. This information is directly
related to our ability in estimating the absolute value of the turbulence
stratification. To better analyse the uncertainties between the MASS and the GS
we took advantage of the availability of measurements taken during the same
campaign by a third independent instrument (DIMM - Differential Imaging Motion
Monitor) measuring the integrated turbulence extended on the whole 20 km. Such
a cross-check comparison permitted us to define the reliability of the
instruments and their measurements, their limits and the contexts in which
their use can present some risk.
