I am interested in locating information of the evidence of z evolution that has been collected of the spectra of relatively high z sources over the previous 75(35) years. Before I get too far into the question, I thought I would try to find what research might have been done on the subject. Unfortunately, I have not been able to find references that seemed to be topical in the NASA database and cant seem to develop a search strategy that doesn't turn up a forest of papers on redshift that are not pertinent to the particular question of the Δz/Δt relationship. At this point, I have yet to take the time yet to do any calculations to determine if it is even feasible to suppose that such information could be gleaned from the body of source data contained in the high z catalogs given the magnitudes involved, etc. I would appreciate any assistance regarding this particular subject and direction as to where I might find additional resources, papers or study reports that address the question. If we take z0 as indicating the instantaneous recessionary speed for a distant source measured at t0 (the point in time that the spectra of the distant source was first recorded), then, we might suppose that if the period t1 - t0 is sufficiently large, one would expect that z1 - z0 >> 0, and thus, an observable. I just haven't been able to find any materials that address the issue. Thank you.