> Why are big centile differences wrong? This is data for weight centile charts, given an age (here in months) and a weight and thirdly male or female. They will give centiles, meaning for that give age and weight and sex x % of children will be below that weight for that age and and the remaining % will be above that weight for that age. If for a given age the weight is slowly supplied higher and higher then the centile values should slowly go up. The data provided is only provided step-wise, so I have to work out interpolating values. This is the full original data set, so not yet normalised: Sex Agemos P3 P5 P10 P25 P50 P75 P90 P95 P97 1 0.0 2.355451 2.526904 2.773802 3.150611 3.530203 3.879077 4.172493 4.340293 4.446488 1 0.5 2.799549 2.964656 3.209510 3.597396 4.003106 4.387423 4.718161 4.910130 5.032625 1 1.5 3.614688 3.774849 4.020561 4.428873 4.879525 5.327328 5.728153 5.967102 6.121929 1 2.5 4.342341 4.503255 4.754479 5.183378 5.672889 6.175598 6.638979 6.921119 7.106250 1 3.5 4.992898 5.157412 5.416803 5.866806 6.391392 6.942217 7.460702 7.781401 7.993878 1 4.5 5.575169 5.744752 6.013716 6.484969 7.041836 7.635323 8.202193 8.556813 8.793444 1 5.5 6.096775 6.272175 6.551379 7.043627 7.630425 8.262033 8.871384 9.255615 9.513307 1 6.5 6.564430 6.745993 7.035656 7.548346 8.162951 8.828786 9.475466 9.885436 10.161350 1 7.5 6.984123 7.171952 7.472021 8.004399 8.644832 9.341490 10.021014 10.453314 10.744924 1 8.5 7.361236 7.555287 7.865533 8.416719 9.081120 9.805593 10.514064 10.965736 11.270838 1 9.5 7.700624 7.900755 8.220839 8.789882 9.476500 10.226124 10.960172 11.428676 11.745385 1 10.5 8.006677 8.212684 8.542195 9.128110 9.835308 10.607722 11.364450 11.847633 12.174357 1 11.5 8.283365 8.495000 8.833486 9.435279 10.161536 10.954660 11.731602 12.227661 12.563083 1 12.5 8.534275 8.751264 9.098246 9.714942 10.458854 11.270871 12.065948 12.573402 12.916450 1 13.5 8.762649 8.984701 9.339688 9.970338 10.730626 11.559963 12.371453 12.889108 13.238933 1 14.5 8.971407 9.198222 9.560722 10.204418 10.979925 11.825241 12.651749 13.178670 13.534622 1 15.5 9.163180 9.394454 9.763982 10.419863 11.209555 12.069725 12.910152 13.445640 13.807244 1 16.5 9.340328 9.575757 9.951840 10.619101 11.422068 12.296170 13.149687 13.693251 14.060194 1 17.5 9.504964 9.744251 10.126434 10.804329 11.619777 12.507080 13.373106 13.924442 14.296548 1 18.5 9.658975 9.901830 10.289680 10.977527 11.804779 12.704728 13.582902 14.141875 14.519093 1 19.5 9.804039 10.050187 10.443295 11.140475 11.978966 12.891168 13.781331 14.347954 14.730343 1 20.5 9.941645 10.190823 10.588812 11.294771 12.144043 13.068254 13.970425 14.544842 14.932559 1 21.5 10.073105 10.325070 10.727592 11.441847 12.301541 13.237653 14.152010 14.734482 15.127765 1 22.5 10.199575 10.454102 10.860844 11.582978 12.452830 13.400856 14.327718 14.918606 15.317770 1 23.5 10.322062 10.578949 10.989635 11.719299 12.599135 13.559197 14.499004 15.098756 15.504178 1 24.5 10.441442 10.700513 11.114904 11.851817 12.741544 13.713860 14.667158 15.276296 15.688406 1 25.5 10.558473 10.819575 11.237473 11.981419 12.881023 13.865896 14.833316 15.452424 15.871699 1 26.5 10.673803 10.936812 11.358059 12.108888 13.018424 14.016230 14.998478 15.628189 16.055142 1 27.5 10.787982 11.052801 11.477280 12.234907 13.154497 14.165673 15.163512 15.804500 16.239671 1 28.5 10.901473 11.168034 11.595670 12.360072 13.289897 14.314932 15.329172 15.982139 16.426090 1 29.5 11.014664 11.282926 11.713683 12.484898 13.425194 14.464622 15.496103 16.161769 16.615076 1 30.5 11.127870 11.397820 11.831705 12.609827 13.560881 14.615270 15.664853 16.343950 16.807196 1 31.5 11.241348 11.513000 11.950054 12.735234 13.697379 14.767324 15.835883 16.529146 17.002914 1 32.5 11.355298 11.628692 12.068997 12.861438 13.835046 14.921166 16.009575 16.717730 17.202598 1 33.5 11.469880 11.745079 12.188748 12.988699 13.974182 15.077109 16.186239 16.910002 17.406538 1 34.5 11.585210 11.862300 12.309477 13.117232 14.115032 15.235412 16.366119 17.106191 17.614947 1 35.5 11.701371 11.980456 12.431315 13.247207 14.257796 15.396279 16.549405 17.306461 17.827972 1 36.0 11.759784 12.039910 12.492682 13.312776 14.329944 15.477724 16.642369 17.408165 17.936247 2 0.0 2.414112 2.547905 2.747222 3.064865 3.399186 3.717519 3.992572 4.152637 4.254922 2 0.5 2.756917 2.894442 3.101767 3.437628 3.797528 4.145594 4.450126 4.628836 4.743582 2 1.5 3.402293 3.547610 3.770157 4.138994 4.544777 4.946766 5.305632 5.519169 5.657379 2 2.5 3.997806 4.150639 4.387042 4.784820 5.230584 5.680083 6.087641 6.332837 6.492574 2 3.5 4.547383 4.707123 4.955926 5.379141 5.859961 6.351512 6.802770 7.076723 7.256166 2 4.5 5.054539 5.220488 5.480295 5.925888 6.437588 6.966524 7.457119 7.757234 7.954730 2 5.5 5.522500 5.693974 5.963510 6.428828 6.967850 7.530180 8.056331 8.380330 8.594413 2 6.5 5.954272 6.130641 6.408775 6.891533 7.454854 8.047178 8.605636 8.951544 9.180938 2 7.5 6.352668 6.533373 6.819122 7.317373 7.902436 8.521877 9.109878 9.476009 9.719621 2 8.5 6.720328 6.904886 7.197414 7.709516 8.314178 8.958324 9.573546 9.958480 10.215388 2 9.5 7.059732 7.247736 7.546342 8.070932 8.693418 9.360271 10.000792 10.403355 10.672801 2 10.5 7.373212 7.564327 7.868436 8.404400 9.043262 9.731193 10.395451 10.814695 11.096070 2 11.5 7.662959 7.856916 8.166069 8.712513 9.366594 10.074306 10.761063 11.196246 11.489076 2 12.5 7.931030 8.127621 8.441460 8.997692 9.666089 10.392576 11.100887 11.551451 11.855391 2 13.5 8.179356 8.378425 8.696684 9.262185 9.944226 10.688742 11.417920 11.883477 12.198293 2 14.5 8.409744 8.611186 8.933680 9.508085 10.203294 10.965321 11.714911 12.195223 12.520785 2 15.5 8.623887 8.827638 9.154251 9.737329 10.445406 11.224627 11.994379 12.489340 12.825610 2 16.5 8.823370 9.029399 9.360079 9.951715 10.672507 11.468779 12.258624 12.768249 13.115272 2 17.5 9.009668 9.217980 9.552723 10.152900 10.886386 11.699718 12.509741 13.034150 13.392045 2 18.5 9.184160 9.394782 9.733630 10.342415 11.088682 11.919211 12.749637 13.289041 13.657990 2 19.5 9.348127 9.561110 9.904140 10.521669 11.280895 12.128870 12.980037 13.534728 13.914971 2 20.5 9.502760 9.718170 10.065489 10.691956 11.464397 12.330156 13.202503 13.772841 14.164667 2 21.5 9.649162 9.867081 10.218820 10.854462 11.640434 12.524391 13.418439 14.004844 14.408584 2 22.5 9.788355 10.008874 10.365180 11.010271 11.810139 12.712769 13.629108 14.232045 14.648068 2 23.5 9.921281 10.144499 10.505533 11.160373 11.974537 12.896363 13.835635 14.455614 14.884316 2 24.5 10.048808 10.274829 10.640760 11.305666 12.134555 13.076132 14.039025 14.676585 15.118389 2 25.5 10.171734 10.400664 10.771667 11.446967 12.291025 13.252935 14.240165 14.895872 15.351221 2 26.5 10.290791 10.522736 10.898985 11.585013 12.444692 13.427531 14.439838 15.114276 15.583627 2 27.5 10.406644 10.641710 11.023380 11.720466 12.596223 13.600594 14.638729 15.332495 15.816318 2 28.5 10.519902 10.758192 11.145454 11.853924 12.746209 13.772713 14.837433 15.551131 16.049903 2 29.5 10.631115 10.872728 11.265746 11.985917 12.895172 13.944404 15.036464 15.770701 16.284906 2 30.5 10.740782 10.985812 11.384743 12.116919 13.043572 14.116113 15.236260 15.991643 16.521765 2 31.5 10.849350 11.097886 11.502878 12.247348 13.191808 14.288225 15.437192 16.214324 16.760846 2 32.5 10.957219 11.209345 11.620537 12.377573 13.340229 14.461065 15.639570 16.439044 17.002447 2 33.5 11.064747 11.320538 11.738058 12.507913 13.489134 14.634908 15.843647 16.666048 17.246809 2 34.5 11.172248 11.431774 11.855740 12.638647 13.638774 14.809981 16.049626 16.895526 17.494115 2 35.5 11.279999 11.543323 11.973842 12.770013 13.789365 14.986470 16.257667 17.127623 17.744504 2 36.0 11.334045 11.599289 12.033121 12.835999 13.865074 15.075293 16.362500 17.244688 17.870885 Hopefully this makes it clearer. RBS