  We review the tools and procedures for the analysis of the data collected by
X-ray photoelectric gas polarimeters, like the ones on-board the Imaging X-ray
Polarimetry Explorer (IXPE). Although many of such tools are in principle
common with polarimeters working at other energy bands, the peculiar
characteristics and performance of these devices require a specific approach.
We will start from the analysis of the raw data read-out from this kind of
instruments, that is, the image of the track of the photoelectron. We will
briefly present how such images are processed with highly-specialized
algorithms to extract all the information collected by the instrument. These
include energy, time of arrival and, possibly, absorption point of the photon,
in addition to the initial direction of emission of the photoelectron. The last
is the quantity relevant for polarimetry, and we will present different methods
to obtain the polarization degree and angle from it. A simple method, used
extensively especially during the development phase of X-ray photoelectric gas
polarimeters, is based on the construction and fitting of the azimuthal
distribution of the photoelectrons. We will discuss that there are several
reasons to prefer an analysis based on Stokes parameters, especially when one
wants to analyze measurements of real, i.e., not laboratory, sources. These are
quantities commonly used at all wavelengths because they are additive, and then
operations like background subtraction or the application of calibration are
trivial to apply. We will summarize how Stokes parameters can be used to adapt
current spectroscopy software based on forward folding fitting to perform
spectro-polarimetry. Moreover, we will derive how to properly associate the
statistical uncertainty on a polarimetry measurement and the relation with
another statistical indicator, which is in the minimum detectable polarization.
