title: Microsoft Excel. Piece-wise Least-Squares Fit with Solver. My excel sheet produces right answers sometimes, wrong answers other times
body: <p>I am trying to do a non-linear regression with data I have for my research. Since it is nonlinear, I can't use Simplex LP. Instead I was doing GRG Nonlinear with upper and lower bounds on all parameters. </p>
  
  <p>It is weird because Excel produces answers that are right sometimes, and other times it is wrong. I have to manually change the parameters to arbitrary numbers, run Solver again, and hope that it is right. Let me show you my excel sheet.</p>
  
  <p><a href="https://drive.google.com/a/case.edu/file/d/0Bw0aJV0lW2eTaHFRUFhobVZ4NWs/edit?usp=sharing" rel="nofollow">https://drive.google.com/a/case.edu/file/d/0Bw0aJV0lW2eTaHFRUFhobVZ4NWs/edit?usp=sharing</a></p>
  
  <p>Basically, I am looking to two linear lines to fit the data I have. The raw data can be divided into two portions, both linear. The point which the two lines cross is the Critical Value.</p>
  
  <p>The correct output with my raw data is where the Critical Value = 0.006707. The last time I ran it, which is on the excel sheet, you can see that Critical Value = 3.36E-06. </p>
  
  <p>If it would help you to understand this better in context, I am measuring surface tension of various systems. The critical value is so called Critical Micelle Concentration in my field.</p>
  
  <p>Thanks Guys.</p>