Interaction

It must be stressed again that the no-interaction model does not always fit factorial data. Sometimes the effect of one factor is different for different levels of the others. Interactions in models with 3 or more factors are however difficult to understand, so we only briefly mention their existence here.

Shrimp culture

Scientists conducted a factorial experiment to assess how shrimp growth was affected by temperature, salinity and the density of shrimps in the tanks. The table below shows the average 4-week gain in weight per shrimp (mg) from the post-larval stage for each combination of factor levels.

  80 shrimps/litre   160 shrimps/litre
Salinity 25°C 35°C   25°C 35°C
10% 73 349   86 364
25% 482 330   208 316
40% 397 205   243 281