  Between 1995 and 2009, electron temperature (Te) measurements of more than
15000 plasmas produced in the Joint European Torus (JET) have been carefully
reviewed using the two main diagnostics available over this time period:
Michelson interferometer and Thomson scattering systems. Long term stability of
JET Te is experimentaly observed by defining the ECE TS ratio as the ratio of
central Te measured by Michelson and LIDAR.
  This paper, based on a careful review of Te measurement from 15 years of JET
plasmas, concludes that JET Te exhibits a 15-20% effective uncertainty mostly
made of large-scale temporal drifts, and an overall uncertainty of 16-22%.
  Variations of 18 plasma parameters are checked in another data set, made of a
"reference data set" made of ohmic pulses as similar as possible between 1998
and 2009. Time drifts of ECE TS ratios appear to be mostly disconnected from
the variations observed on these 18 plasma parameters, except for the very low
amplitude variations of the field which are well correlated with off-plasma
variations of a 8-channel integrator module used for measuring many magnetic
signals from JET.
  From mid-2002 to 2009, temporal drifts of ECE TS ratios are regarded as
calibration drifts possibly caused by unexpected sensitivity to unknown
parameters; the external temperature on JET site might be the best parameter
suspected so far.
  Off-plasma monitoring of MI made of calibration performed in the laboratory
are reported and do not appear to be clearly correlated with drifts of ECE TS
ratio and variations of magnetics signals integrators. Comparison of
estimations of plasma thermal energy for purely Ohmic and NBI-only plasmas does
not provide any definite information on the accuracy of \mi or \lidar
measurements.
  Whatever causes these Te drifts, this experimental issue is regarded as
crucial for JET data quality.
