For nonexperimental data, causal direction can often be inferred if information about time is available. This is because (according to many, though not all, theories) causes must precede their effects temporally. This can be determined by statistical time series models, for instance, or with a statistical test based on the idea of Granger causality , or by direct experimental manipulation. The use of temporal data can permit statistical tests of a pre-existing theory of causal direction. For instance, our degree of confidence in the direction and nature of causality is much greater when supported by cross-correlations , ARIMA models, or cross-spectral analysis using vector time series data than by cross-sectional data .
The theory of "karma and rebirth" raises numerous questions—such as how, when, and why did the cycle start in the first place, what is the relative Karmic merit of one karma versus another and why, and what evidence is there that rebirth actually happens, among others. Various schools of Hinduism realized these difficulties, debated their own formulations, some reaching what they considered as internally consistent theories, while other schools modified and de-emphasized it, while a few schools in Hinduism such as Carvakas, Lokayatana abandoned "karma and rebirth" theory altogether.    Schools of Buddhism consider karma-rebirth cycle as integral to their theories of soteriology .