Area probability sample design procedures for the National Children's Study required contiguous strata and clusters that maximized within-stratum and within-cluster homogeneity while maintaining approximately equal size of the strata or clusters.
A possible reason is that the regression models were more effective in the Green Hills stands, thus requiring fewer strata to obtain the same level of efficiency.
Thirteen patients and six health professionals were invited to participate in the investigation after responding to the study advertisement (and being deemed appropriate in meeting one of the required purposive sampling strata).
In calculating the total sampling effort required in each stratum for a target CV of 25% [36] we used encounter rates of elephant dung and great ape nests in each of the survey strata from prior baseline data collected independently in each of the management units [19], [48], [49].
The number of sets of radiographs required in each stratum was 25.
However, this included any evidence of supranuclear ophthalmoplegia, whereas allocation to the PSP stratum required significant supranuclear impairment of downward gaze.
We invited all eligible older people residing in each block selected to participate and then moved to the next block, repeating the invitation process, until we gained the sample required for that stratum.
Thus, a minimum sample size of 270 was required from the three strata.
We sought to delineate 18 strata as a reasonable number to capture overall heterogeneity; each stratum was required to have an MOS within 10% of the county MOS divided by the number of strata (2,113 based on 2001 2008 data, giving an acceptable MOS range for each stratum of 1,902 2,324).
Although the actual decomposition of temperature and ozone effects is more complex, this example demonstrates the stronger assumptions taken on when estimating effects of complex, multivariable exposures (compared with the total effects, which require positivity across strata of confounders but not across strata of the mediator, ozone).
