There seems to be a bug in the star formation rate density calculation routine. The volume is only the real 'proper volume' at the current redshift and not at any other redshifts, because it uses the current redshift in the calculation of proper volume in all redshifts. #--------------------------------- from yt.mods import * from yt.analysis_modules.star_analysis.api import * import pylab pf=load('HD1200/HD1200') dd=pf.h.all_data() sfr = StarFormationRate(pf, data_source=dd, bins=10) sfr.write_out(name="StarFormationRate.out") time, lookback, z, Msolyr, MsolyrMpc3, Msol, cumMsol=na.loadtxt('StarFormationRate.out',usecols=(0,1,2,3,4,5,6),skiprows=2,unpack=True) cMpc3,=pylab.plot(z,Msolyr/(pf['CosmologyComovingBoxSize']/pf['CosmologyHubbleConstantNow'])**3) currentz,=pylab.plot(z,MsolyrMpc3/(1+pf.current_redshift)**3+1e-5) Expect,=pylab.plot(z,MsolyrMpc3/(1+z)**3) pylab.legend([cMpc3,currentz,Expect], ['[s$^{-1}$cMpc$^{-3}$]', 'use only current z to fit', 'Expected to fit']) pylab.savefig('SFRD.eps') #--------------------------------- in the file: yt/analysis_modules/star_analysis/sfr_spectrum.py line 151: self.Msol_yr_vol.append(self.mass_bins[i] / \ (self.time_bins_dt[i] * tc / YEAR) / vol) The vol is a fixed number instead of varying by the redshift, even though the numerator in solar masses per year is from different redshifts. So I think a couple lines above it should create an empty array of vol vol = [] In the loop for i, time... calculate the right volume vol[i] Then instead of dividing by the constant vol, divide by vol[i] corresponding to the right redshift. self.Msol_yr_vol.append(self.mass_bins[i] / \ (self.time_bins_dt[i] * tc / YEAR) / vol[i]) From G.S.