The reliability and stability of a quantity-frequency method and a diary method of measuring alcohol consumption.

The study aimed to assess the test-retest reliability of two commonly used measures of alcohol consumption, the quantity-frequency (QF) method and the diary method, as well as the stability of scores on the two measures over time. Two methods of assessing reliability and stability were employed. The first was a traditional method based on calculation of correlation coefficients for agreement between scores on repeated measures over a short retest interval to yield test-retest reliability coefficients, and over a long retest interval to yield stability coefficients. The second method was that devised by Wiley and Wiley (1970) to differentiate the effects of reliability and stability on repeated measures over time. The two methods were applied to a sample of heavy drinkers and to a sample of light drinkers. The results indicated that both the QF and diary measures are reliable in measuring alcohol consumption of light drinkers. Both measures are less reliable for heavy drinkers. The results indicate, in addition, that drinking consumption levels of light drinkers demonstrate a high degree of stability. However, the consumption levels of heavy drinkers demonstrate less stability, especially over a long time period. Heavy drinkers significantly reduced reported levels of alcohol consumption on both measures after the first test, suggesting a regression to the mean effect or the possibility of unintended intervention effects due to repeated measurement of drinking behaviour.