Statistical Process Chart (SPC) comparing monthly and mean contaminated BC rate for the pre-study period compared with the study period. Statistical Process Chart (SPC) comparing monthly and mean contaminated BC rate for the pre-study period compared with the study period. James Bentley et al. BMJ Qual Improv Report 2016;5:u206760.w2754 © 2016, Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/