Choosing between MPFIT functions

	MPFIT	MPFITFUN (1D)/ MPFIT2DFUN (2D)	MPFITEXPR
Required?	YES	No	No
How does it work?	User passes an IDL function which computes "data minus model" residuals.	User passes the data, and an IDL function which computes the model.	User passes the data, and a string containing an IDL expression which computes the model.
Easy to Use?	No	Yes	Yes
Must compile separate IDL function?	Yes	Yes	No
Speed	Fastest	Faster	Fast
Control over fitting process	Most	Some	Some
Meant to be used interactively?	No	Yes	Yes
Parameter bounds and "freezing"?	Yes	Yes	Yes