To deal with visit missingness, we partitioned the sample into four strata according to which of the age 7 and age 11 visits were attended.
If strata composition data are unavailable, some indication of pollution potential can be obtained from considerations of the proximity of worked strata to marine beds (which typically have high pyrite contents).
The association of pollution with mortality can be thought to be inferred from the extent to which WITHIN STRATA daily death counts are explained by daily exposure concentrations.
Such studies are important in assessing environmental risks associated with mining areas in the WPRA, where Se-bearing rocks have been displaced from the stable, slowly weathering strata, to the surficial environment in which weathering processes are much more rapid.
Participants will be stratified into 16 strata according to two factors: K6 score (5 or greater or less than 5) in the baseline survey and the eight workplaces to which they belong.
However, no detail is given on how information was exploited to define the strata, which seems to be the result of expert interpretation.
This category is defined by the low temperatures observed during the winter, which cause the trees in the upper strata to lose their leaves because of physiologic drought caused by the cold weather (11 ).
All studies, regardless of statistical method, require the assumption that we have measured all confounders, that is, that the variables measured are sufficient to create strata in which survival time if never exposed is statistically independent of observed exposure.
Few offers have been made for space in buildings under strata title, which is estimated to cover about two-thirds of the new buildings opening this year and 70 percent of those opening in 2011.
Furthermore, this can easily lead to a large number of strata (e.g. 10 centres, and three binary covariates would lead to 10×2×2×2 = 80 strata), which increases the chances of some strata being dropped from the analysis.
