Calibration code

Can anyone help, I am trying to create a code that will allow me to calibrate 2000 data points that correspond to a time of 0 to 4 secs. I want to create a linear regression using only .5 to .7 secs of the data. I tried using polyfit but it just isnt working, so any ideas?

[the cyclist]

It's not possible to diagnose a problem when you only say "it just isnt working". Are you able to post your code, along with a more detailed description of what you think is not correct about the results?

[Walter Roberson]

Do you have evidence that the response curve is polynomial ?

0.5 seconds of (2000 points / 4 seconds) works out to be 250 points. What polynomial order are you using to fit those 250 points?

[Sean de Wolski]

And even if those 250 points are linear, remember the dangers of extrapolating the others:

Mark Twain says in Life on the Mississippi (1884):

“In the space of one hundred and seventy six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over a mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oölitic Silurian Period, just a million years ago next November, the Lower Mississippi was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-pole. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo [Illinois] and New Orleans will have joined their streets together and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.”