# NonlinearModelFit returning unexpected result

I am attempting to perform a weighted fit of some data. The Fit function produces the correct quadratic fit, which I am fine with, but it does not appear to support weighted fitting. Therefore, I had to use the LinearModelFit and NonlinearModelFit functions. The linear model performed as expected, but the nonlinear model outputted a very large array of nonsensical results (eg a=0,b=0,c=0.003). Attempting to fit a linear model using the function also failed, with the same huge array of results.

What am I doing wrong with the functions? I have tried to follow the examples as closely as I could, but still cannot get it to work.

alist = {{75, 0.}, {76, 3/5}, {77, 9/11}, {78, 6/7}, {79, 8/13}, {80,
2/3}, {81, 5/7}, {82, 7/9}, {83, 6/7}, {84, 5/9}, {85, 1/4}, {86,
4/7}, {87, 2/3}, {88, 1}, {89, 1}, {90, 1/2}, {91, 3/7}, {92,
1}, {93, 5/6}, {94, 2/3}, {95, 2/5}, {96, 3/5}, {97, 5/7}, {98, 4/
7}, {99, 6/7}, {100, 3/4}, {101, 5/7}, {102, 9/10}, {103, 7/
10}, {104, 5/6}, {105, 5/8}, {106, 3/7}, {107, 11/15}, {108, 13/
18}, {109, 9/11}, {110, 11/14}, {111, 8/9}, {112, 7/8}, {113, 2/
3}, {114, 9/10}, {115, 10/13}, {116, 9/11}, {117, 8/11}, {118, 5/
6}, {119, 11/13}, {120, 5/7}, {121, 14/19}, {122, 7/11}, {123, 13/
17}, {124, 17/21}, {125, 11/13}, {126, 15/17}, {127, 7/11}, {128,
17/19}, {129, 15/17}, {130, 15/19}, {131, 9/11}, {132, 7/8}, {133,
26/27}, {134, 23/24}, {135, 24/25}, {136, 29/31}, {137, 29/
34}, {138, 17/20}, {139, 30/37}, {140, 31/36}, {141, 16/17}, {142,
22/23}, {143, 33/35}, {144, 1}, {145, 7/8}, {146, 1}, {147,
1}, {148, 1}, {149, 7/8}, {150, 1}};
wlist = {0, 5, 11, 7, 13, 6, 7, 9, 7, 9, 4, 7, 3, 2, 1, 4, 7, 3, 6,
12, 5, 5, 7, 7, 7, 8, 14, 20, 10, 18, 16, 7, 15, 18, 11, 14, 9, 8,
9, 10, 13, 11, 11, 6, 13, 7, 19, 11, 17, 21, 26, 17, 11, 19, 17,
19, 22, 16, 27, 24, 25, 31, 34, 40, 37, 36, 51, 23, 35, 22, 24, 31,
27, 26, 8, 54};
linmod = LinearModelFit[alist, x, x, Weights -> wlist]
NonlinearModelFit[alist, ax^2 + bx + c, {a, b, c}, x,
Weights -> wlist]
linmod2 =
NonlinearModelFit[alist, ax + b, {a, b}, x, Weights -> wlist]

• Seems like a typographical problem. Beware of the difference:ax^2 + bx + c and a x^2 + b x + c: I get the expected result when typing in the quadratic function with spaces between a,b and x Feb 11, 2015 at 15:21

linmod  = LinearModelFit[alist, x, x, Weights -> wlist];
quadmod = NonlinearModelFit[alist, a x^2 + b x + c, {a, b, c}, x,  Weights -> wlist];
linmod2 = NonlinearModelFit[alist, a x + b, {a, b}, x, Weights -> wlist];

`