I am doing data analysis for an Atomic Absorption Spectroscopy experiment, and the question I have is pretty basic.
I have done a linear regression on my data, and used the LINEST function in Excel to get s_y, s_m, and s_b. Since I am interested in the value of the x-intercept, it was easy to propagate the values from the LINEST function to find the s value for that value. My problem lies in finding the 95% CI for the x-intercept.
I took replicate data for each of the points I plotted, so each one is actually the average of 3 measurements under the same conditions. Since I had 5 sets of conditions, there were 15 points total going into this analysis. Does this mean I must divide the s value I propagated by sqrt(15) and multiply it by the appropriate t-value for the 95% CI with 13 degrees of freedom? Or have I gotten lost somewhere?