The Activity Index (AI) is a well-known index for comparing the contribution of different entities on various fields, for example scientific articles from different countries on various subjects, but it lacks important properties. We will show, that the AI is a log-normal distribution and that it is common in literature to transform the AI by the logarithm to a normal distribution. Last, we will present an alternative transformation special for longitudinal data, that transforms the AI to a normal distribution, too, without the negative properties of the logarithm.