Estimates of sexual partnership dynamics: extending negative and positive gaps to status lengths.

BACKGROUND: The aims of this study are to generalise the concept of gap length between partners and to estimate the duration of four types of measures for heterosexual partnerships, called status lengths: (1) time spent as single before becoming monogamous (S-M, positive gap), (2) duration of concurrency before monogamy (C-M, negative gap), (3) duration of monogamy before concurrency (M-C) and (4) time spent in monogamy before becoming single (M-S). METHODS: Medians and CIs were obtained using the US National Survey of Family Growth Cycle 6 conducted in 2002. RESULTS: A significant gender difference was found in the monogamous to single status length (medians 38 months for women, and 19.3 months for men). Other status lengths were similar between genders (S-M: 20 women, 18 men; M-C: 16 women, 13 men; and C-M: 5 for women and men). Respondents younger than the median age at first marriage showed shorter status lengths compared to older ones. Median status lengths were comparable between heterosexuals and bisexuals. Percentage of concurrency in 1 year was 3.3% for women and 3.8% for men. CONCLUSIONS: One of the new status lengths (M-C) qualitatively indicates the transmission risk to an upcoming concurrent partner. The set of four status lengths may be useful in the context of epidemiological models with partnership dynamics.

On a new method to study drastic change in ecosystems.

We describe a novel method to study drastic change in ecosystems, based on generic functions used in the study of phase transitions and related physical phenomena. We illustrate its use by applying it to the problem of shallow lake eutrophication, and express our results in terms of an interplay between phosphorus content in the water column and fluxes of this substance between the lake and both its biological community and its surroundings. We contrast our solution to this problem with a previous one based on the concept of resilience, and on bifurcation analysis of a dynamical equation that also involves phosphorus concentration and fluxes. We then suggest a generalized dynamical scheme incorporating the generic functions above, that reduces to our original method in the stationary condition, and allows in addition dealing with cyclic and chaotic regimes, as illustrated through a particular example.