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1. Introduction
According to the latest reports of World Health Organization (WHO) uncorrected refractive errors are the most common cause of visual impairment worldwide, accounting for 43% of cases and representing an important cause of blindness [1]. Uncorrected refractive errors have also been associated with reduced vision-related quality of life and with loss of independence [2,3]. The estimated global cost of lost productivity due to refractive error vision impairment in 2007 was more than 200 billion United States dollars [4]. Most of this could be eliminated simply with refraction and appropriate vision correction [5,6].
The prevalence of refractive errors has been reported to vary with race, age, gender and geographic regions. Population-based data indicate the prevalence of myopia as being higher in children of Chinese ethnicity; but in Chinese adults the rate of myopia is not much higher than what is found in White adult population [2]. Environmental factors like level of education, occupation, near-work load, time outdoors as a child are also associated with aetiology of refractive errors [2,7,8,9]. The gender differences in the prevalence of refractive errors have been also reported, but many studies have failed to confirm these associations [7,10,11,12,13,14].
During the last two decades several studies concerning the prevalence of refractive errors in Asia [12,13,14,15,16,17,18,19], Australia [20,21] and North America [2,9,22,23,24] have been undertaken. However there are very few from Europe and all are from the Western part [7,11,25,26]. Poland is the biggest eastern European country, with a population of 38 million people according to the 2011 national census [27]. Due to a lack of data from Poland and other post-Soviet nations, we conducted an epidemiological survey on a sample population of older adults in the city of Lodz, which results have recently been published [6,28]. The aim of the present study was to investigate the distribution of refractive errors and their characteristics in this population.
2. Materials and Methods
2.1. Subjects, Eye Examinations and Definitions
The study design was a cross-sectional study. The sampling and recruitment methods for this study have been described in details in our previous papers [6,28]. Sample size for the study was calculated with 99% confidence, within an error bound of 5%. The sample size requirement was 661, as calculated by: [ Formula omitted. See PDF....