5.2. Trend analysis
Trend analysis was conducted using control charts and seasonal indices, as opposed to traditional inferential statistical methods due to the limitations of our sampling approach in terms of randomness (as detailed in Section 2.1). As generally seen in most surveillance activities of food bacterial pathogens, the large number of samples with negative results represents a challenge in conducting trend analyses, as a very large sample size is required to observe any potentially significant events or trends. Nonetheless, the methods developed and presented in this paper proved to be easy to use, interpret and replicate, and provided a good overview of the data at hand. These methods helped identify some trends worth noting and exploring further, notably in leafy vegetables, the group withthe largest sample size (n 1⁄4 12,073). The control chart for leafy vegetables showed spikes of concern in levels of bacterial contamination over the summer (e.g., July) for multiple years of the study. The global seasonal index (combined data for all six bacteria) and the seasonal index for generic E. coli alone confirmed that there was a significantly higher rate of bacterial contamination in July, and that this trend is driven by the results for generic E. coli. Further analyses by commodity sub-groups showed that this trend was evident in the organically-grown leafy vegetable group and not in the conventionally-grown leafy vegetables. Due to the more com- mon usage of animal manure as fertilizer in organic production, the microbial safety of organic produce (in comparison with conventionally-grown produce) has been questioned over the years. Several studies on this subject (Mukherjee, Speh, Dyck, & Diez-Gonzalez, 2004; Oliveira et al., 2010; Tango, Choi, Chung, & Oh, 2014) found that the hygienic quality of produce was impacted by organic production and that organic produce was more sus- ceptible to fecal contamination. Our findings support this obser- vation and indicate that summertime is the prime period to observe this difference in hygienic quality in organic leafy vegetables.