Academic journal article Bulletin of the World Health Organization

Field Trial of Applicability of Lot Quality Assurance Sampling Survey Method for Rapid Assessment of Prevalence of Active Trachoma

Academic journal article Bulletin of the World Health Organization

Field Trial of Applicability of Lot Quality Assurance Sampling Survey Method for Rapid Assessment of Prevalence of Active Trachoma

Article excerpt

Introduction

According to WHO estimates, 5.6 million people are blind or severely visually impaired because of trachoma, and 146 million people currently have active disease (1). Resource constraints in countries in which trachoma is endemic mean that interventions for trachoma should be as cost effective as possible. A method of identifying areas with high prevalences of trachoma so that resources can be used where they are most needed is thus a necessary component of trachoma control strategies. The trachoma rapid assessment (TRA) method was developed for this purpose. It is a survey method designed to determine whether trachoma is a public health problem in a community. It is rapid in terms of the time needed to collect and analyse data and simple in that local staff are able to conduct assessments without external assistance. The method uses a two-stage sample. The selection of communities to be sampled and the selection of people within communities are both biased optimally towards selecting those at highest risk (2). The second stage of TRA does nol use a standardized sampling procedure. People are sampled with the subjective judgement of members of the survey team. This sampling method cannot be relied upon to estimate or classify prevalence of trachoma. At best, TRA can indicate the presence or absence of trachoma in a community. Despite this, data from TRA surveys are taken consistently to indicate prevalence and used to make comparisons between communities.

A recent assessment of TRA compared the results of two TRA surveys with door-to-door surveys of the same communities. The results of the TRA surveys were not consistent with each other or with the door-to-door surveys in their estimates of the prevalence of active trachoma (3). The ability of TRA to classify reliably by prevalence communities in which trachoma is endemic, rather than distinguishing between communities in which active trachoma is present and those in which active trachoma is probably absent, is doubtful. A survey method that is reliable, rapid, and simple enough to be performed by staff currently employed in TRA surveys is needed. Sequential sampling methods produce reliable results with small sample sizes (that is, they have the potential to be used as rapid methods) (4). The method that has received most attention for public health applications is lot quality assurance sampling (LQAS), which is used widely in the manufacturing industry to judge the quality of a lot (batch) of items. In this context, LQAS is used to identify lots that are likely to contain an unacceptably large number of defective items. In the public health context, LQAS may be used to identify communities with low levels of service coverage or high prevalences of disease. Lot quality assurance sampling produces data that are easy to analyse. Data analysis is performed as the data is collected and consists of counting the number of "defects" (for example, children with active trachoma or incomplete vaccination records) in the sample and checking whether a predetermined number has been exceeded. Most public health applications of LQAS have focused on use of the method to evaluate service delivery (5-11). Less work has been conducted on its use to assess disease prevalence (11-15).

Data from LQAS are collected and analysed with a sampling plan that specifies a maximum sample size and the number of defects that are allowed in the sample before a community is classified as "high prevalence". Sampling plans are developed by specifying a classification system (that is the levels of prevalence that define situations of high and low prevalence) and acceptable probabilities of error. The use of a sampling plan in the field is straightforward. Sampling stops when the maximum sample size is met or the number of defects allowed in the sample is exceeded. If the maximum sample size is met without the number of defects allowed in the sample being exceeded, the community is classified as low prevalence. …

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