Linear trendline error

[vvorland]

Hi, I've plotted a graph showing a valve opening vs flow rate. I've then inserted a linear trendline - pretty simple stuff really. My problem occured when I changed the y-axis with the x-axis. (i.e plotting flow rate vs valve opening) The trendline does not give the same result as before!??

Originally the trendline crossed the y-axis at ~2000m3/h (indicating 2000m3/h flow at 0% opening) but when I swap axis, the trendline crosses the y-axis at 15% opening (indicating that 0 flow at 15%open)

How is this possible?

I've got a large amount of data, apprx 26000 datapoints, is it possible that excel does not manage to use all of them?

Any thoughts are appriciated

 

[vvorland]

Thanks to this website I've actually found the solution.

EXCEL will not provide an unbiased fit of the data between two sets of assay results. The reason for this is that the software assumes one variable is independent, and the other is dependent. For example, the independent variable might be a setting on an instrument, and the dependent variable might be a measurement of output. The underlying assumption is that the independent variable is precisely known while the output is not.

Regards
V.