Mustafa -

This is not my subject matter area, but I assume you are talking about "simple linear regression," but I'm not sure if it is applicable to a range which includes the origin. If it is, then your reasoning that your line must have a zero intercept sounds like a good one.

Is there any subject matter knowledge reason for this not to be a straight line? Are there range limitations? I'm wondering if dealing with percents might cause you any problem??

You could look at a graphical residual analysis to see if your model looks good. You could plot the estimated residuals on the y-axis, and the predicted values on the x-axis.

Best wishes - Jim

Mustafa -

If you are looking at sulphate to sulphur ratios which change spatially, then you appear to need a more complex model.

Jim

Hi Jim, thanks for answers,

Actually I do not consider the values which are below a cutoff (2pct by mass S) and extremely high values for analysis. In this case, the plot seems to have a linear trend. When I apply a linear trendline result is SulphateSulphur(pct)= 0.8xSulphur(pct) - 1. It says 80% of total S belongs to sulphate molecules but what does fixed value 1 mean here? Another problem is number of S measurements are very high around lower values. I think they dominate the trendline.

For spatial distribution, I am wondering If I can plot the distribution of (SSpct+1)/(Spct) ratios over an area and to classify possible domains using statistical tools. Thats why i am aligned to choose linear regression.

Other than that, If you suggest a better statistical approach, it will be very much appreciated

Best regards