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Sampling Guidelines for Surveillance and Vector-Control Activities Related to Chagas Disease

(Antônio Carlos Silveira & Odécio Sanches, December 2003)

Full Text (available in PDF in Spanish and Portuguese [±42 pp, ±350 Kb]; chapter headings translated below for user orientation)

I. Introduction

II. Entomological research using sampling: Situations in which it would be indicated; Parameters and Indicators
II.1. Establishing baselines
II.2. Assessing impact of the household chemical treatment, routine follow-up, and surveillance
II.3. Assessing achievement of the goal of interrupting vector-borne transmission
II.4. Assessing achievement of the goal of eliminating the vector

III. Seroprevalence surveys using sampling: Situations in which they would be indicated; Parameters and Indicators
III.1. Establishing baselines
III.2. Assessing impact on control
III.3. Demonstrating the interruption of transmission

IV. Sampling
IV.1. General comments: Probability sample and why it should be used
IV.2. Some plans for simple probability sampling
IV.2.i. Simple random sampling
IV.2.ii. Systematic sampling
IV.2.iii. Simple accidental stratified sampling
IV.2.iv. Sampling by clusters
IV.3. The statistical issue of sample size
IV.3.i. Is there a sample size that is infallible?
IV.3.ii. What is the intention here?
IV.3.iii. Statistical parameters set for constructing tables on sample size
IV.4. Sampling Plans
IV.4.i. Estimating Indices of Dispersion (I.D.), Infestation (I.I.), and Seroprevalence (P) for establishing baselines
IV.4.ii. Estimating Indices of Dispersion (I.D.) and Infestation (I.I.) in routine operations and surveillance
IV.4.iii. Estimating Index of Dispersion (I.D.) in negative localities of areas where control measures are being implemented; Existing information; Illustration by example
IV.4.iv. Estimating Index of Infestation (I.I.) in areas where control measures are being implemented; Simple random sampling (or equivalent) of households in order to estimate infestation in a given locality
IV.4.v. Estimating Seroprevalence (P) in areas being controlled in serological surveys; Estimating Seroprevalence (P) of human infection in assessing impact of control and demonstrating the interruption of transmission

V. Practical application of sampling techniques in Chagas disease control programs

VI. Appendices
1: Entomological indicators in everyday use
2: Utilizing a telephone directory for substituting a Random Digits Table

VII. Bibliographical References
Recommended Bibliography
Classic and Full Texts for Initial Study

PAHO Chagas Page

Introduction

The purpose of any statistical study is to gather information on a given population. Once a population is large enough for it to be possible or justifiable—in most cases—to study it comprehensively, any conclusions should be derived from the examination of a mere part of it.

Thus, sampling surveys are used to seek information on populations or specific population groups, for which a total-coverage survey or census is impracticable, whether by virtue of the size of the population or by virtue of its being dispersed over large areas, which would demand too much time and financial cost.

In monitoring and controlling vector-borne transmission of Chagas disease, there are various situations in which sampling techniques can offer sufficient and reliable information when it comes to guiding action. This means greater operational rationality and cost reduction. In practice, this type of approach is not infrequent; but it is seldom done using secure epidemiological and statistical bases.

Operational rationality basically depends on the selectivity of interventions. The degree of possible selectivity is limited because current techniques used in entomological research offer low sensitivity—above all in situations in which vector density is low or minimal, as tends to happen in advanced control phases. It is indispensable that this assumption be considered in planning disease-control activities, in the scope of case-finding, and in the proof of surveillance of the household vector.

Because vector detection depends almost always on a critical density of existing populations, monitoring areas considered at a given time to be vector-free but nonetheless considered at risk—by virtue of the type of existing habitat, its physical proximity to infested areas, or other conditions—is absolutely compulsory. Furthermore, it does not make sense to keep under routine intervention and comprehensive coverage all household units from all localities that show up as negative in census-based entomological surveys.

Also in the case of seroprevalence studies used to scale the problem or performance auditing, it is not justifiable to include the entire population.

These are the reasons justifying the proposal for and production of these Sampling Guidelines for Chagas disease surveillance and vector control.

The complete planning and execution of a sampling survey is a complex of activities that begins with defining objectives, population of interest, and parameters to be estimated (via descriptive surveys) or compared (via analytical surveys); defining and specifying instruments for measurement and/or observation; forming and training field teams; supervision; and logistics, in addition to planning and administrative follow-up.

With regard to the requirement for simplicity and briefness in this guide—which is intended for unrestricted use by operational personnel from control programs, and not for specialists in technical aspects and sampling techniques—the guide will not address the broad complexity of probabilistic sampling surveys. Hence, the guide avoids such subjects as the presentation of simple results or expressions to estimate standard error, which are necessary for constructing confidence intervals or for carrying out hypothetical testing.

Due to the intentional simplicity of this text, it does not cite specific references on sampling aspects. For readers or users interested in expanding their knowledge of the content of this guide, a set of recommended publications is included at the end for eventual consultation.