Table 2 Comparison of alert threshold development techniques
From: Early detection of malaria foci for targeted interventions in endemic southern Zambia
Technique | Advantages | Disadvantages | |
|---|---|---|---|
WHO[17] | Upper third quartile of monthly case numbers from preceding 5 years | • Calculation does not require a computer • Not skewed by epidemic years | • Requires 5 years of historic data • Limited utility for local public health response when calculated at the monthly and district-wide level |
Cullen[18] | Monthly mean number of cases + 2 standard deviations from 5 years of historic data where "epidemic years" have been excluded | • Simple calculation | • Requires 5 years of historic data • Limited utility for local public health response when calculated at the monthly and district-wide level • Exclusion of "epidemic years" is arbitrarily defined |
C-sum[19] | Mean number of cases for a given month, the preceding month and the subsequent month from the past 5 years plus 2 standard deviations (note: the same technique has been applied to weekly data for a variety of diseases including malaria [23]). | • Smooths fluctuations due to irregular reporting rather than disease incidence by providing a larger 15 historic months sample size. | • Requires 5 years of historic data • Limited utility for local public health response when calculated at the monthly and district-wide level |
Poisson[20] | Upper 95% confidence interval limit of Poisson distribution based on weekly case numbers from past 2 or more years of historic data at sites grouped by transmission zones and adjusted by population of catchment areas. | • Granular weekly and local thresholds better reflect the seasonal and geographic variations and allow for more agile public health responses • Does not require as many years of historic data • A larger historic sample size is obtained by grouping sites with similar observed patterns of transmission | • Greater influence of "epidemic years" on mean and threshold calculations because fewer years of historic data are used • Questionable applicability of Poisson assumptions • Zonal thresholds introduce an aggregation bias with inconsistent sensitivities between RHCs within a zone |