the nature of the questions in the survey greatly influence outcome. Multiple particularly broad question databases have failed to produce the Big Five as the top five traits. In many studies, the five factors are not fully orthogonal to one another; that is, the five factors are not independent. Orthogonality is viewed as desirable by some researchers because it minimizes redundancy between the dimensions. This is particularly important when the goal of a study is to provide a comprehensive description of personality with as few variables as possible. Methodological issues Factor analysis, the statistical method used to identify the dimensional structure of observed variables, lacks a universally recognized basis for choosing among solutions with different numbers of factors. A five factor solution depends on some degree of interpretation by the analyst. A larger number of factors may underlie these five factors. This has led to disputes about the "true" number of factors. Big Five proponents have responded that although other solutions may be viable in a single data set, only the five-factor structure consistently replicates across different studies. Surveys in studies are often online surveys of college students. Results do not always replicate when run on other populations or in other languages. Moreover, the factor analysis that this model is based on is a linear method incapable of capturing nonlinear, feedback and contingent relationships between core systems of individual differences. Theoretical status A frequent criticism is that the Big Five is not based on any underlying theory; it is merely an empirical finding that certain descriptors cluster together under factor analysis. Although this does not mean that these five factors do not exist, the underlying causes behind them are unknown. Jack Block's final published work before his death in January 2010 drew together his lifetime perspective on the five-factor model. He summarized his critique of the model in terms of: the atheoretical nature of the five-factors. their "cloudy" measurement. the model's inappropriateness for studying early childhood. the use of factor analysis as the exclusive paradigm for conceptualizing personality. the continuing non-consensual understandings of the five-factors. the existence of unrecognized but successful efforts to specify aspects of character not subsumed by the five-factors. He went on to suggest that repeatedly observed higher order factors hierarchically above the proclaimed Big Five personality traits may promise deeper biological understanding of the origins and implications of these superfactors. Evidence for six factors rather than