If you would be willing to share the SAS code, maybe I could clarify things.

With two factors A and B, there is a main effect of A, a main effect of B and an interaction effect of A and B. I don't know about the combination of A and B you are talking about. Explain.

I think something is wrong in your usage of SAS or your understanding of the result. The reason is because interaction I think is the same as the combination of both treatments. Hence you may have; Main effects A, B, and the interaction effect A*B which is the combination of A and B.

Jos Feys

data two_factor_lettuce;

input Block$ Iron$ season$ trt_comb$ FSW TFW ;

datalines;

1 EDTA 1 EDTA1 131.44 4206.2

2 EDTA 1 EDTA1 141.00 4512.0

3 EDTA 1 EDTA1 136.33 4362.7

1 EDTA 2 EDTA2 105.73 3383.2

2 EDTA 2 EDTA2 99.50 3184.0

3 EDTA 2 EDTA2 74.10 2371.2

1 DTPA 1 DTPA1 140.98 4511.3

2 DTPA 1 DTPA1 151.83 4858.7

3 DTPA 1 DTPA1 146.82 4698.3

1 DTPA 2 DTPA2 93.73 2999.2

2 DTPA 2 DTPA2 77.34 2474.9

3 DTPA 2 DTPA2 100.05 3201.6

;

Proc print;

proc glm;

class Block Iron season trt_comb;

model FSW TFW = Block Iron season Iron*season;

means Iron season Iron*season/LSD;

run;

proc glm;

class Block trt_comb;

model FSW TFW = Block trt_comb ;

means trt_comb/LSD;

run;

Godwin Lebari Tuaneh

You can check

data two_factor_lettuce;

input Block$ Iron$ season$ trt_comb$ FSW TFW ;

datalines;

1 EDTA 1 EDTA1 131.44 4206.2

2 EDTA 1 EDTA1 141.00 4512.0

3 EDTA 1 EDTA1 136.33 4362.7

1 EDTA 2 EDTA2 105.73 3383.2

2 EDTA 2 EDTA2 99.50 3184.0

3 EDTA 2 EDTA2 74.10 2371.2

1 DTPA 1 DTPA1 140.98 4511.3

2 DTPA 1 DTPA1 151.83 4858.7

3 DTPA 1 DTPA1 146.82 4698.3

1 DTPA 2 DTPA2 93.73 2999.2

2 DTPA 2 DTPA2 77.34 2474.9

3 DTPA 2 DTPA2 100.05 3201.6

;

Proc print;

proc glm;

class Block Iron season trt_comb;

model FSW TFW = Block Iron season Iron*season;

means Iron season Iron*season/LSD;

run;

proc glm;

class Block trt_comb;

model FSW TFW = Block trt_comb ;

means trt_comb/LSD;

run;

For "TFW" as well as for "FSW", this gives:

Effect----------df---------- F value------------------- p-value

Block -----------2 -------------0.10 ------------------- 0.9049

Iron --------------1------------- 0.28 --------------------0.6146

season ---------1 -----------48.79-------------------- 0.0004

Iron*season -1 ------------ 0.84 --------------------0.3951

So, there is only a sign. effect of season not of Iron, nor of the interaction.