Texas Instruments TI-83 PLUS - Manual (Page 367)

TI-83 Plus Statistics 364
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Plot the data and trace to determine the x-distance between the
beginning and end of N complete periods, or cycles. Then divide the
total distance by N.
After your first attempt to use SinReg and the default value for iterations to
fit the data, you may find the fit to be approximately correct, but not
optimal. For an optimal fit, execute
SinReg 16, Xlistname , Ylistname ,2 p / b
where b is the value obtained from the previous SinReg execution.