Although a number of life-log tools for health care have been proposed, they share a common problem; the drop-out problem of achieving long-term continuous usage in users. Providing numerical data is believed to be useful in tackling the problem, but, that can also have adverse effects if people overly react or misinterpret the meaning of the data. To investigate how people react to numerical data, we asked 817 undergraduate students to evaluate some fictitious tools for four domains (e.g., blood pressure) with one of five different designs; with standard or precise figures (with two additional digits), combined with/without interpretive messages for the data (e.g. “good condition”), or no numerical data (message only). The results indicate that involvement in health-care activities is independent of tool evaluation, such that only tool evaluations were strongly influenced by the existence of numerical data. The merits and demerits of numerical presentation are discussed.