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 blockII
III IV V 22 24 21 23 27 25 26 28 29

6. Add a random variation component of up to 10%II
III IV V
0.10
1.52
2.18
1.92
1.21
1.90
2.44
1.36
0.18
1.71
1.96
0.55
2.41
2.20
2.94
1.45
2.50
1.67
0.86
1.13
0.47
0.04
2.64
0.73
2.87
2.79
0.95
1.28
0.82
1.55
2.04
1.70
0.43
2.97
2.10
0.32
0.75
0.77
2.84
2.96

7. Next add on the effect of a soil fertility gradientII
III IV V 1 2 3 4

8. What follows are things we may not have thought of!!
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 soilII
III IV V -2 -4 -6

10. Some years ago, somebody had cleared a house site in the middle of our trial areaII
III IV V -5

11. We had neglected to plan for adequate planting material and had to get what we could findII
III IV V
-4.86
-0.48
-4.54
-2.17
-3.05
-1.58
-4.36
-0.66
-1.84
-5.69
-0.89
-1.01
-5.28
-1.69
-1.39
-1.67
-1.95
-7.25
-0.80
-0.86
-0.01
-5.39
-1.92
-0.56
-5.89
-3.80
-2.22
-4.19
-2.95
-1.16
-4.12
-1.21
-3.76
-0.10
-5.34
-0.71
-5.23
-3.04
-1.24
-2.42
-1.17
-4.30

12. Weather got too wet and we had to dig drains quickly to avoid water logging
5.00

13. Final yield results of the trial
22.24
31.04
23.64
26.75
28.16
25.31
22.08
29.23
21.52
19.49
33.82
32.94
29.28
33.73
30.81
27.26
22.50
26.16
38.70
38.81
24.86
16.74
23.56
29.56
26.15
31.97
30.93
30.65
28.60
28.00
28.11
23.70
32.34
35.18
34.94
25.36
27.63
25.58
31.40
25.09
29.71
34.97
29.35
33.67
29.66

14. Analysis of results with all the unexpected results
Randomized Complete Block
Source DF SS MS F P
BLOCK
TREAT
Error
Total 44
Grand Mean CV 15.93
Result is not significant- ie we cannot find any differences between any of the cultivars, even with five replicates. Note the CV is quite reasonable at 16 %.

15. Had we managed to avoid the drains
Randomized Complete Block
Source DF SS MS F P
BLOCK
TREAT
Error
Total 44
Grand Mean CV 13.06
LSD All-Pairwise Comparisons
TREAT Mean Homogeneous Groups A AB AB AB AB BC BC C C

16. Had we also managed to have prepared good planting material
Randomized Complete Block
Source DF SS MS F P
BLOCK
TREAT
Error
Total 44
Grand Mean CV 8.17
LSD All-Pairwise Comparisons
TREAT Mean Homogeneous Groups A AB AB BC
BCD
CDE
CDE DE E

17. And avoided the old house site
Randomized Complete Block
Source DF SS MS F P
BLOCK
TREAT
Error
Total 44
Grand Mean CV 5.63
LSD All-Pairwise Comparisons
TREAT Mean Homogeneous Groups A AB BC C CD CD DE DE E

18. And the area of red soil !!! Randomized Complete Block
Source DF SS MS F P
BLOCK
TREAT
Error
Total 44
Grand Mean CV 3.04
LSD All-Pairwise Comparisons
TREAT Mean Homogeneous Groups A B C C D DE EF F G

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 …