Hi Skya, I'm not convinced there's anything strange about this. You're only reporting the best fit in each case. Presumably the fit series using at most 4 standards also generates the result of standard 1, 2, and 3 being a possibility, and generates the same statistical measures of fit as in the 3 standard case. The moral is that the BEST fit out of linear combinations is not all that meaningful if there are multiple other fits of similar statistical quality. Linear combination fitting can be used to narrow down the possibilities, but as always shouldn't be used as a "black box." (Of course, if one of the 4-standard fits has much better statistical quality than any of the 3-standard fits, that might suggests that there really ARE 4 components in the sample!) --Scott Calvin Sarah Lawrence College
I'm doing linear combination fits using 10 standards total, and choosing to use at most 3 or 4 standards in the fit. The problem is that depending on which I choose, 3 or 4, the results vary significantly. Often one standard will show up in one fit using at most 3 standards and not in a fit using at most 4 standards. For example:
Unknown A:
Fit using at most 3 standards Standard 1= 0.213 Standard 2= 0.374 Standard 3= 0.413
Fit using at most 4 standards Standard 1= 0.282 Standard 2= 0.269 Standard 4= 0.102 Standard 5= 0.346
Unknown B:
Fit using at most 3 standards Standard 1= 0.278 Standard 2= 0.745
Fit using at most 4 standards Standard 2= 0.694 Standard 3= 0.117 Standard 4= 0.227
Note that the same 10 standards are run in each case, its only the number I choose to use in the best fit that is varying. As you can see, hopefully, the results vary significantly. Any ideas as to why this might happen? Any advice you can give on attaining more consistent results would be greatly appreciated.
Thank you!
Skya Fawcett PhD Candidate Department of Geological Sciences and Geological Engineering Queen's University, Kingston, Canada