The design and analysis of variety trials using mixtures of composite and individual plot samples
Field trials for evaluating plant varieties are conducted to obtain information on a range of traits including grain yield and grain quality. Some grain quality traits are costly to measure so it is not possible to test all plots individually. It has therefore become common practice to test a single composite sample for each variety in the trial but this precludes the use of an efficient statistical analysis. We propose an approach in which a proportion of varieties is tested by using individual replicate samples. The remaining varieties may either be tested as composite samples or with reduced replication. This allows application of efficient mixed model analyses for both the inexpensive traits (using data from the complete set of individual replicate plots) and expensive traits (using data from a subset of the plots or a mixture of composite and individual plot samples).