To use Shewhart control charts to compare variability in inadequacy rates from Papanicolaou (Pap) and liquid-based cytology (LBC).Design
Retrospective analysis of quality assurance data.Setting
Eleven Welsh cytology laboratories.Methods
Shewhart ‘p’ charts were plotted for proportions of slides reported as inadequate. Charts were compared for statistical control.Main outcome measures
Evidence of statistical control in the processes.Results
Control charts allowed easy interpretation of patterns in the data. Variability in inadequacy rates was much lower for LBC than for Pap cytology.Conclusion
Monitoring inadequate rates with Shewhart charts provides more information than tabular monitoring reports, assisting in quality improvement. With respect to inadequacy rates, LBC is less variable than Pap cytology.