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The final oscillation analysis of the complete set of data collected by CHORUS in the years 1994-1997 is presented. Reconstruction algorithms of data extracted by electronic detectors were improved and the data recorded in the emulsion target were analysed by new automated scanning systems, allowing the use of a new method for event reconstruction in emulsion. CHORUS has applied these new techniques to the sample of 1996-1997 events for which no muons were observed in the electronic detectors. Combining the new sample with the data analysed in previous papers, the overall sensitivity of the experiment to the ν τ appearance is thus improved. In a two-neutrino mixing scheme, a 90% C.L. upper limit of sin 2 2θ µτ < 4.4 × 10 −4 is set for large ∆m 2 , improving by a factor 1.5 the previously published CHORUS result.3