## 1 Introduction  
Classification studies are a valuable source of information in various areas of science. The problem of classification is related to the problem of similarity of objects. Objects arranged in simple, one-dimensional sets may be classified in a unique way according to one, properly chosen, aspect of similarity. The problem becomes more complicated if we consider multidimensional sets, i.e., objects characterized by several different aspects. The degree of similarity depends on the selected aspects, on the number of aspects considered and on the mathematical measure establishing the relations between different properties. One of class of objects considered by us is biological sequences. Both graphical and numerical classification of these objects is possible using methods based on _Graphical Representations_[1, 2]. Within these methods, one can create a large number of different types of numerical characteristics (descriptors) of the plots representing the sequences. One kind of descriptors we propose are the distribution moments related to different statistical distributions describing the DNA sequences. We have shown that using these descriptors a pair of the sequences that differ by only one base can be distinguished. The coordinates of the descriptors representing these sequences are different in the classification maps [1]. The distribution moments we have also introduced as new descriptors of another class of objects--the molecular spectra [3, 4]. The applications of the theory of molecular similarity are broad. Except for the studies of the properties of the systems explicitly considered in our works, the new descriptors may have broad range of interdisciplinary applications. For example, they may be applied in computational pharmacology and toxicology [5]. Our new descriptors have also found their application in the classification of the solutions in the chaotic systems [6], or in the classification of the stellar spectra [7; 8]. Another kind of descriptor we propose are values used in the classical dynamics such as coordinates of center of mass or the moments of inertia. Examples of the classification studies using these descriptors may be found in the theory of molecular similarity [9] or in bioinformatics [10]. A class of objects considered in this work are groups of individuals. The studies are focused on the _retirement threshold_. A graphical representation of the results known as the _Correspondence Analysis_ (CA) proves to be very useful in this kind of study [11]. Recently, CA was applied for studies on a variety of problems, for example, on high school dropouts [12], and also in archeology [13], in food science [14], etc. In the CA the information about the whole system is stored on maps in which objects under consideration are represented by points located in a specific way. The classification of the objects is here performed by studying distances between the points and, in particular, by identifying clusters of the points. Objects corresponding to the points which form a cluster are similar in some way. Progress of medicine and lower fertility rate caused significant changes in the structure of modern societies. The number of seniors in developed countries is growing dynamically. In Poland in 2010 the percentage of people aged 65 and over was 19%. According to Eurostat forecasts, in 2030 the ratio of elderly people to the population aged 15-64 will be 36%, and in 2050--56% [15]. Due to acceleration of aging process, it seems reasonable to study the quality of life of older people. An important role in shaping the quality of life of seniors plays the retirement threshold, which is described in literature as symbolic moment--starting a new chapter in life. It involves many negative changes such as loss of professional status, deterioration of the economic situation, as well as reduction in the number of social interactions. On the other hand, pensioners have much free time for family life and hobby.