***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: PURPOSE To describe the prevalence of hyperopia and associated factors in a representative sample of Australian schoolchildren 6 and 12 years old . DESIGN Population -based cross-sectional study . PARTICIPANTS Schoolchildren ages 6 ( n = 1765 ) and 12 ( n = 2353 ) from 55 r and omly selected schools across Sydney . METHODS Detailed eye examinations included  cycloplegic autorefraction , ocular biometry , cover testing , and dilated fundus examination .  Information on birth and medical history were obtained from a parent question naire . MAIN OUTCOME MEASURES Moderate hyperopia defined as spherical equivalent ( SE ) refraction of > or = + 2.00 diopters ( D ) , and eye conditions including amblyopia , strabismus , astigmatism , and anisometropia . RESULTS Prevalences of moderate hyperopia among children ages 6 and 12 were 13.2 % and 5.0 % , respectively ; it was more frequent in children of Caucasian ethnicity ( 15.7 % and 6.8 % , respectively ) than in children of other ethnic groups . Compared with children without significant ametropia ( -0.49 < or = SE refraction < or = + 1.99 D ) , the prevalence of eye conditions including amblyopia , strabismus , abnormal convergence , and reduced stereoacuity was significantly greater in children with moderate hyperopia ( all Ps < 0.01 ) . Maternal smoking was significantly associated with moderate hyperopia among 6-year-olds ( P = 0.03 ) , but this association was borderline among 12-year-olds ( P = 0.055 ) . Early gestational age ( < 37 weeks ) and low birth weight ( < 2500 g ) were not statistically significant predictors of moderate hyperopia in childhood . CONCLUSIONS Moderate hyperopia was strongly associated with many common eye conditions , particularly amblyopia and strabismus , in older children . Birth parameters did not predict moderate hyperopia PURPOSE To determine the prevalence of refractive error types in Singaporean Chinese children aged 6 to 72 months . METHODS The Strabismus , Amblyopia and Refractive Error in Singaporean Children ( STARS ) is a population -based study in southwest Singapore . Door-to-door recruitment of participants was used , with disproportionate r and om sampling in 6-month increments . Parental question naires were administered . Participant eye examinations included  logMAR visual acuity , cycloplegic autorefraction , and ocular biometry .  Overall and age-specific prevalences of myopia ( spherical equivalence [ SE ] < or= -0.50 D ) , high myopia ( SE < or= -6.00 D ) , hyperopia ( SE > or= + 3.00 D ) , astigmatism ( cylinder > or= + 1.50 D ) , and anisometropia ( SE difference between each eye > or=2.00 D ) were calculated . RESULTS A total of 3009 children were examined ( participation rate , 72.3 % ) . Right eye ( OD ) cycloplegia data were available for 1375 boys and 1264 girls ( mean age , 41 months ) . Mean OD SE was + 0.69 D ( SD 1.15 ) . Overall myopia prevalence was 11.0 % with no variance between the sexes ( P = 0.91 ) . The prevalence of high myopia ( at least -6.00 D ) was 0.2 % . The prevalences of hyperopia , astigmatism , and anisometropia were 1.4 % , 8.6 % , and 0.6 % , respectively . Most astigmatism ( > 95 % ) was with-the-rule ( cylinder axes between 1 degrees and 15 degrees or 165 degrees and 180 degrees ) . Myopia was present in 15.8 % , 14.9 % , 20.2 % , 8.6 % , 7.6 % , and 6.4 % of children aged 6 to 11 , 12 to 23 , 24 to 35 , 36 to 47 , 48 to 59 , and 60 to 72 months , respectively . Prevalence increased with age for astigmatism ( P < 0.001 ) , but not for hyperopia or anisometropia ( P = 0.55 and P = 0.37 ) , respectively . CONCLUSIONS The prevalences of myopia and astigmatism in young Singaporean Chinese children are high , but that of hyperopia is low . Age effects were observed for each refractive error category , but differences between the sexes were not significant . Age-related variation in myopia prevalence may be influenced by ocular development , environment , and /or testability PURPOSE To describe the methodology , sampling strategy and preliminary results for the Aston Eye Study ( AES ) , a cross-sectional study to determine the prevalence of refractive error and its associated ocular biometry in a large multi-racial sample of school children from the metropolitan area of Birmingham , Engl and . METHODS A target sample of 1700 children aged 6 - 7 years and 1200 aged 12 - 13 years is being selected from Birmingham schools selected r and omly with stratification by area deprivation index ( a measure of socio-economic status ) . Schools with pupils predominantly ( > 70 % ) from a single race are excluded . Sample size calculations account for the likely participation rate and the clustering of individuals within schools . Procedures involve st and ardised protocol s to allow for comparison with international population -based data .  Visual acuity , non-contact ocular biometry ( axial length , corneal radius of curvature and anterior chamber depth ) and cycloplegic autorefraction  are measured in both eyes . Distance and near oculomotor balance , height and weight are also assessed . Question naires for parents and older children will allow the influence of environmental factors on refractive error to be examined . RESULTS Recruitment and data collection are ongoing ( currently N=655 ) . Preliminary cross-sectional data on 213 South Asian , 44 black African Caribbean and 70 white European children aged 6 - 7 years and 114 South Asian , 40 black African Caribbean and 115 white European children aged 12 - 13 years found myopia prevalence of 9.4 % and 29.4 % for the two age groups respectively . A more negative mean spherical equivalent refraction ( SER ) was observed in older children ( -0.21 D vs + 0.87 D ) . Ethnic differences in myopia prevalence are emerging with South Asian children having higher levels than white European children 36.8 % vs 18.6 % ( for the older children ) . Axial length , corneal radius of curvature and anterior chamber depth were normally distributed , while SER was leptokurtic ( p<0.001 ) with a slight negative skew . CONCLUSIONS The AES will allow ethnic differences in the ocular characteristics of children from a large metropolitan area of the UK to be examined . The findings to date indicate the emergence of higher levels of myopia by early adolescence in second and third generation British South Asians , compared to white European children . The continuation of the AES will allow the early determinants of these ethnic differences to be studied PURPOSE To assess the prevalence of refractive errors and vision impairment in school-age children in Shunyi District , northeast of Beijing , the Peoples Republic of China . METHODS R and om selection of village-based clusters was used to identify a sample of children 5 to 15 years of age .  Resident registration books were used to enumerate eligible children in the selected villages and identify their current school . Ophthalmic examinations were conducted in 132 schools on children from 29 clusters during May 1988 to July 1998 , including  visual acuity measurements , cycloplegic retinoscopy , cycloplegic autorefraction , ocular motility evaluation , and examination of the external eye , anterior segment , media , and fundus .  Independent replicate measurements of all children with reduced vision and a sample of those with normal vision were done for quality assurance monitoring in three schools . RESULTS A total of 6,134 children from 4,338 households were enumerated , and 5,884 children ( 95.9 % ) were examined . The prevalence of uncorrected , presenting , and best visual acuity 0.5 ( 20/40 ) or worse in at least one eye was 12.8 % , 10.9 % , and 1.8 % , respectively ; 0.4 % had best visual acuity 0.5 or worse in both eyes . Refractive error was the cause in 89.5 % of the 1,236 eyes with reduced vision  , amblyopia in 5 % , other causes in 1.5 % , with unexplained causes in the remaining 4 % . Myopia -0.5 diopter or less in either eye was essentially absent in 5-year-old children , but increased to 36.7 % in males and 55.0 % in females by age 15 . Over this same age range , hyperopia 2 diopters or greater decreased from 8.8 % in males and 19.6 % in females to less than 2 % in both . Females had a significantly higher risk of both myopia and hyperopia . CONCLUSIONS Reduced vision because of myopia is an important public health problem in school-age children in Shunyi District . More than 9 % of children could benefit from prescription glasses . Further studies are needed to determine whether the upward trend in the prevalence of myopia continues far beyond age 15 and whether the development of myopia is changing for more recent birth cohorts purpose To determine the age- and gender-specific prevalences of refractive errors in Tehran through a population -based study . methods A total of 6497 citizens representing a cross-section of the population of Tehran were sample d from 160 clusters using a stratified , r and om , cluster sampling strategy . Eligible people were enumerated through a door-to-door household survey in the selected clusters and were invited to participate . All participants were transferred to a clinic for an extensive eye examination and interview . Refractive error was determined using manifest and cycloplegic refraction . Myopia was defined as the spherical equivalent of −0.5 diopters ( D ) or more and hyperopia was defined as the spherical equivalent of more than + 0.5 D. results Of those sample d , 4565 ( 70.3 % ) people participated in the study . Refraction data for 4354 participants aged five years and over are presented . The age- and gender-st and ardized prevalence of myopia based on manifest refraction was 21.8 % ( 95 % confidence interval [ CI ] , 20.1 to 23.5 ) and that for hyperopia was 26.0 % ( 95 % CI , 24.5 to 27.6 ) . The prevalences based on cycloplegic refraction  were 17.2 % ( 95 % CI , 15.6 to 18.8 ) and 56.6 % ( 95 % CI , 54.7 to 58.6 ) , respectively . Prevalences of myopia and hyperopia differed significantly among the age and gender groups ( P < 0.001 ) . Astigmatism of 0.75 cylinder diopter or greater was present in 29.6 % ( 95 % CI , 28.0 to 31.3 ) of right eyes with manifest refraction and in 30.3 % ( 95 % CI , 28.5 to 32.1 ) with cycloplegic refraction . Among the study population , 6.1 % ( 95 % CI , 5.3 to 6.8 % ) had anisometropia of 1 D or more . conclusions This report has provided details of the refractive status in the population . We have documented prevalences of myopia , hyperopia , astigmatism and anisometropia by age and gender , identifying more affected age- and gender-groups for prevention programs in the community Objective : This study investigated the distribution pattern of refractive status and prevalence of refractive errors in school-age children in Western China to determine the possible environmental factors . Methods : A r and om sampling strategy in geographically defined clusters was used to identify children aged 6 - 15 years in Yongchuan , a socio-economically representative area in Western China . We carried out a door-to-door survey and actual eye examinations , including  visual acuity measurements , stereopsis examination , anterior segment and eyeball movements , fundus examinations , and cycloplegic retinoscopy with 1 % cyclopentolate . Results : A total of 3469 children living in 2552 households were selected , and 3070 were examined . The distributions of refractive status were positively-skewed for 6 - 8-year-olds , and negatively-skewed for 9 - 12 and 13 - 15-year-olds . The prevalence of hyperopia ( ≥+2.00 D spherical equivalent [ SE ] ) , myopia ( ≤-0.50 D SE ) , and astigmatism ( ≥1.00 diopter of cylinder [ DC ] ) were 3.26 % , 13.75 % , and 3.75 % , respectively . As children 's ages increased , the prevalence rate of hyperopia decreased ( P<0.001 ) and that of myopia increased significantly ( P<0.001 ) . Children in academically challenging schools had a higher risk of myopia ( P<0.001 ) and astigmatism ( ≥1.00DC , P = 0.04 ) than those in regular schools . Conclusion : The distribution of refractive 
    Output:
    
With regard to associated factors , age has an inverse association with hyperopia .
The frequency of hyperopia is higher among White children and those who live in rural areas .
There is no consensus about the association between hyperopia and gender , family income and parental schooling .