Background: Few studies have assessed the validity or reliability of drinking water intake estimations. In studies investigating potential waterborne disease, this is very important.
Methods: We carried out a validity and reliability study to evaluate water intake estimations. The validity part of the study compared a telephone questionnaire to a diary. The reliability part of the study compared two telephone questionnaires.
Results: 253 participants were recruited. The most commonly consumed water intake categories were plain and boiled tap water. For persons aged less than 12 years, the median volume of plain water consumption was 2.9 glasses/day according to the diary. For persons aged 12 years and over, the median volume of plain water consumption was 3.0 glasses/day and for boiled water 3.0 glasses/day according to the diary. There was negligible systematic over or under reporting by participants for the telephone questionnaire in comparison to the diary and for the comparison of both telephone questionnaires. Water intake estimates were converted to categorical variables and the per cent agreement and weighted kappa statistics were calculated. For both the validity and reliability parts of the study the per cent agreement was 57-78% and the weighted kappa was 0.57-0.81.
Conclusion: Water intake estimates show only moderate levels of validity and reliability when using a telephone questionnaire in comparison to a diary.
Implications: Investigators of potential waterborne disease or nutritional research should exercise some caution in drawing conclusions from minor differences in drinking water intake that are obtained from a telephone questionnaire.