Creating data #

The package includes the function nelder(), which we use to generate data for the examples below. Nelder (1965) suggested a simple notation that could express a large variety of different blocked designs. The notation was proposed in the context of split-plot experiments for agricultural research, where researchers often split areas of land into blocks, sub-blocks, and other smaller divisions, and apply different combinations of treatments. However, the notation is useful for expressing a large variety of experimental designs with correlation and clustering including cluster trials, cohort studies, and spatial and temporal prevalence surveys. We have included the function nelder() that generates a data frame of a design using the notation.

There are two operations:

• > (or $\to$ in Nelder’s notation) indicates “clustered in”.
• * (or $\times$ in Nelder’s notation) indicates a crossing that generates all combinations of two factors.

The function takes a formula input indicating the name of the variable and a number for the number of levels, such as abc(12). So for example ~cl(4) > ind(5) means in each of five levels of cl there are five levels of ind, and the individuals are different between clusters. The formula ~cl(4) * t(3) indicates that each of the four levels of cl are observed for each of the three levels of t. Brackets are used to indicate the order of evaluation.