Location, location, location: Presenting evidence for genetically modified crops

This paper describes the results of a literature survey to determine the most common methods of analysis and measures of data location employed in journal papers that explore the agronomic and economic differences between genetically modified (GM) and non-GM crop varieties. A total of 108 articles were reviewed and the most common measure of location employed was the arithmetic mean. Only a small minority of articles employed the median or stated that the data had been tested for normality. Yet the choice of measure of location can make a difference in terms of presenting differences between GM and non-GM to a readership. This is illustrated using data from one of the first field-level studies of GM crops in Africa (GM cotton in Makhathini Flats, South Africa), where the median actually gave a greater percentage yield advantage for GM over non-GM cotton than did the mean. However while the median could be justified in statistical terms this could be a source of suspicion amongst those who are opponents of GM.

► Large literature on the economic differences between genetically modified (GM) and non-GM crop varieties.
►  Variation in this difference has been identified across crops and region.
► The ways in which data are presented as the basis for the comparison is underexplored in the literature.
►  Differences can depend upon whether the mean or the median is employed.