Greylag geese will retrieve an egg placed outside but near to their nests. It is hypothesised that they are more likely to retrieve large eggs than small eggs. To test this, an ethologist has two decoy eggs made and places one or other at a fixed distance from 30 different nests. Fifteen nests are randomly selected to have the large egg placed by them and another 15 the small egg. The proportion of nests to which each egg is retrieved is calculated.