Speaker
Description
Fisher (1925) introduced the three principles of experimental design: (i) true replicates, (ii) randomization, and (iii) blocking. The former two are strictly required while blocking often increases precision. That is what we tell our agricultural students. However, in practice, randomization is often ignored, either in the first replicate (van Santen and West, 2012) or completely. Often, the design is directly taken from textbooks and thus no randomization at all is performed. In these cases, the same design is sometimes/often repeatedly used across trials and years. The reason for this practice is convenience. Moreover, it allows the first replicate to serve as demonstration plots. A common argument is that the systematic order could have occurred by chance, too. It is currently unclear how large potential effects of the systematic order in series of experiments are, especially a systematic order in the first replicate. We therefore analysed a series of trials conducted at one location across ten years. Eleven or twelve historical winter wheat cultivars were tested each year in four replicates. Depending on the number of cultivars, two different textbook field plans were used across years. Additionally, some recent cultivars were changed over time. However, nine of the cultivars were constant across all ten years, and the order of these cultivars was identical across all years. The current study investigates how large the effects of not randomizing cultivars is. The study shows that mean estimates are biased. Moreover, the variance of treatment comparisons can be under- or over-estimated. The study evaluated the consequences of under- and over estimating treatment comparisons on the chance of selecting the truly best cultivar.
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