Methods for comparing data across differently designed agronomic studies: examples of different meta-analysis methods used to compare relative composition of plant foods grown using organic or conventional production methods and a protocol for a systematic review.

Meta-analyses are methods to combine outcomes from different studies to investigate consistent effects of relatively small magnitude, which are difficult to distinguish from random variation within a single study. Several published meta-analyses addressed whether organic and conventional production methods affect the composition of plant foods differently. The meta-analyses were carried out using different options for the methodology and resulted in different conclusions. The types of designs of field trials and farm comparisons widely used in horticultural and agronomic research differ substantially from the clinical trials and epidemiological studies that most meta-analysis methodologies were developed for. Therefore, it is proposed that a systematic review and meta-analysis be carried out with the aim of developing a consolidated methodology. If successful, this methodology can then be used to determine effects of different production systems on plant food composition as well as other comparable factors with small but systematic effects across studies.