Experimental design is Tab-separated values file. This file has two columns, the first one corresponds to the sample name and the second one corresponds to the phenotype.
sample1 Group_1 sample2 Group_1 sample3 Group_2
Note: In case of paired data the Experimental design file must be ordered.
Here is an example of a file with 4 piared samples (sample1_Normal and sample1_Treated are the same sample before and after treatment):
sample1_Normal Group_1 sample2_Normal Group_1 sample1_Treated Group_2 sample2_Treated Group_2
Here is an other file example see brca_normal-basal_ed.txt.