Introduction > Planning Your Research > Designing Your Experiment > Blocking

Blocking To block an experiment, your treatments should be arranged into groups (blocks). Each block should contain at least one replication of every treatment and each treatment should be represented by the same number of replications. For example, if you're investigating the effects of two concentrations (0.5% and 2%) of three herbicides (Roundup, Garlon and Accord) on the death rate of a single species of weed, each block would contain a weed that had been treated with: 0.5% Roundup 0.5% Garlon 0.5% Accord 2% Roundup 2% Garlon 2% Accord. Blocking is particularly important when external influences, which you have no control over, can bias the results of your experiment. A classic example of this is often found in experiments conducted in glasshouses. Whilst some sections of the glasshouse may be hotter than others due to the arrangement of the heating and ventilation, some sections may receive longer hours of daylight due to the position of the glasshouse in relation to the sun. It is therefore acceptable to block the glasshouse based on four micro-climates: 1) long photoperiod and warm; 2) long photoperiod and cold; 3) short photoperiod and warm and 4) short photoperiod and cold. By designing four blocks, with plants from each treatment replicated in each block, the effect of the micro-climates should be negated.

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