Randomised block design

This type of design is used when the researcher is interested in isolating a major source of variation in addition to the treatment's influence. In the example just given, where three different prices were tested, no allowance was made for the different sizes of the stores, a potentially important influence on the actual sales obtained. The randomised block facilitates the isolation of one extraneous factor from the total experimental error, providing a precise picture of the treatment's actual impact (see Table 9.2). This additional variable has to be identified and measured at the time of the experiment.

Antifreeze sales in supermarkets at various prices

Store size £'000

Price £3.89

Price £4.85

Price £5.89

1 100 +

135

100

60

2 50-100

34

26

15

3 20-50

17

12

7

4 < 20

5

2

1

Total

191

140

83

Here we see how the antifreeze example has been expanded to permit the illustration of a randomised block design. In this case the desire is to control the impact of store size on sales.

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