Hello.  I am trying to figure out how (if?) I can adapt a test of
planned means (Dunnett's test?) to a loess fit.

 

Context:  The experiment consists of growing plants in hydroponic
solution of a period of time (t=0 to crit), after which the nutrient
solution concentration is decreased to a new (lower) level.  Plants are
harvested daily during the course of the experiment and the foliar
nitrogen concentration is measured each day from these destructive
harvests.  The statistical goal is to determine the shortest time
following the change in nutrient solution concentration at which one can
detect a significant (p=0.05) decrease in the foliar nitrogen
concentration.  Therefore, I wish to compare the predicted foliar N
concentration at t=crit to the foliar N concentrations at time t=crit+1,
+2, ..  Because the time course of N does not follow any simple
parametric function, I am using smoothing splines via loess.

 

Is there any way to adapt a test of planned means (say, Dunnett's test)
to this problem?  If not, can anyone suggest how this problem can be
tackled via loess?

Any suggestions are gratefully welcomed.

 

Bill Shipley

 


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