1. VARIABILITY IN TRIALS Adapted fr M Gunther

2. Types of variability • Normal random variability • Controllable variability e.g. soil fertility gradients • Uncontrollable variability e.g. theft of produce • Unexpected variability e.g. digging a drain without thinking of the consequences

3. How to deal with variability • Random variability can be a problem if there are limited plots – increase replication may help • Trial design e.g. blocking may help overcome Controllable variability • Uncontrollable variability may need data manipulation at analysis – missing plots, extrapolation from remaining portions • Uncontrollable variability may be eased by careful site inspection, early planning, regular visits to the trial etc

4. Some experiences I have had • The following set of slides represents a hypothetical variety trial • It has nine treatments with five replications – this would often be considered to be over replicated

5. Basic yield of the varieties, randomised in each block

6. Add a random variation component of up to 10%

7. Next add on the effect of a soil fertility gradient

8. Effects so far are what we could call normal and controllable variability What follows are things we may not have thought of!!

9. One corner of our trial extended onto an area of poor red soil

10. Some years ago, somebody had cleared a house site in the middle of our trial area

11. We had neglected to plan for adequate planting material and had to get what we could find

12. Weather got too wet and we had to dig drains quickly to avoid water logging

13. Final yield results of the trial

14. Analysis of results with all the unexpected results

15. Had we managed to avoid the drains

16. Had we also managed to have prepared good planting material

17. And avoided the old house site

18. And the area of red soil !!!

19. This may be an unreal result But, we can overcome some of these issues with good forward planning, careful site selection, close supervision of trials and …