International Journal of Epidemiology, Vol 26, 1049-1054, Copyright © 1997 by International Epidemiological Association
H Limburg, R Kumar, A Indrayan and KR Sundaram
AIM: To find an optimal cluster size and number of clusters for a
reasonable estimate of the prevalence of cataract blindness in people aged
> or = 50 years in 19 rural districts of a state in India. MATERIALS:
Cluster sampling methodology was used in 19 rural districts of Karnataka
State, India. In each district, 15 clusters were randomly selected and 90
people aged > or = 50 years were examined in each cluster. As a result
the visual acuity and lens status of a total of 22,218 people were
assessed. METHODS: For each district, the design effect for cluster size
ranging from 20 to 90 was calculated and the optimal cluster size and the
required number of clusters to achieve an accuracy of 1% errors and 80%
confidence was assessed. RESULTS: The age and gender adjusted prevalence of
cataract blindness varied from 1.58% to 7.24%, which justifies district
level surveys. The design effect is nearly 1.5 for clusters of sizes 30 and
40. With an average prevalence of 4.93% with 1% error and 80% confidence
level, the optimal number of clusters is 37 and 28 for a cluster size of 30
and 40 respectively and the average sample size for a district around 1100.
CONCLUSIONS: Rapid assessments for cataract blindness in those aged > or
= 50 years can be conducted at district level in India with existing
resources and at affordable costs. These provide reliable data, essential
for effective monitoring and planning. Other parameters, for instance,
surgical coverage can also be assessed. The availability of standardized
software for data entry and analysis and strict adherence to survey
procedures is essential.
ARTICLES
Rapid assessment of prevalence of cataract blindness at district level
Division of Biostatistics and Medical Informatics, University College of Medical Sciences, Delhi, India.
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