Villa 1998.

Methods	FLUOROSIS STUDY Country of study: Chile Geographic location: Rancagua (non‐F), Santiago (low‐F), La Serena (medium‐F), San Felipe and Iquique (high‐F) Year of study: 1996 Year of change in fluoridation status: fluoride was naturally occurring Study design: cross‐sectional study
Participants	Inclusion criteria: lifetime residents of study areas; children aged 7,12 and 15 years in selected schools in study areas Exclusion criteria: none stated Other sources of fluoride: not stated Social class: children selected from schools graded according to socioeconomic status to give similar socioeconomic distribution in each study area Ethnicity: not stated Residential history: lifetime residents Other confounding factors: temperature
Interventions	All natural fluoridation Group 1: 0.07 ppm Group 2: 0.21 ppm Group 3: 0.55 ppm Group 4: 0.93 ppm Group 5: 1.10 ppm
Outcomes	Dental fluorosis (Deans Index); caries data also evaluated within the study but excluded from review due to study design Age at assessment: 15 years
Funding	Study was supported by the Chilean Council for Scientific and Technological Research (FONDECYT) through grant no. 1960993
Notes	Data extracted Villa 1998 differs from that presented in CRD review
Risk of bias
Bias	Authors' judgement	Support for judgement
Sampling	Low risk	Selection of schools for each community was made at random from the complete list of private schools and publicly supported elementary schools. All eligible children were invited to participate
Confounding	High risk	Did not account for the use of fluoride from other sources
Blinding of outcome assessment (detection bias) All outcomes	High risk	Insufficient information
Incomplete outcome data (attrition bias) All outcomes	Low risk	Data presented for all participants
Selective reporting (reporting bias)	Low risk	Data not in suitable format for analysis
Other bias	High risk	There may have been misclassification bias as fluorosis prevalence was reported without taking 'questionable' fluorosis prevalence into account