2.1.2 Temporal Design
2.2.2.3 Panels
The allocation of sampling effort at sites across years can be thought of in terms of groups of sites that have the same revisit pattern across years (i.e., across temporal units) (McDonald, 2003). The set of sites with the same revisit pattern is called a panel. For example, a set of sites visited each year comprises one panel. Some sites visited each year and some sites visited every two years yields two panels, i.e., sites visited in year 1, 2, 3, … make up one panel; sites visited in years 1, 3, 5, 7… make up the second panel. We could introduce a third panel, visited in years 2, 4, 6, 8, etc.
In designing a monitoring program it is as important to establish an efficient temporal design as it is a spatial design. The selection of the panel design depends on objectives, and for particular objectives some panel designs yield greater precision than others. Evaluating the trade-offs among panel designs and objectives is an important part of developing an optimum temporal design. For example, in multiple site surveys, sampling different sites provides a broad description of the variety of conditions in the region of interest. However, if trend detection is a primary objective, revisiting sites each year is likely the most sensitive design for detecting trends in early years of a survey. But, over longer time frames, a design that includes several panels can provide approximately the same trend detection power as a single annual panel (Urquhart and Kincaid, 1999). For example, a design that includes an annual panel and panels that are sampled on a multiple year cycle, such as a panel starting in year 1, covering years 4,7,10,...; another panel starting in year 2, covering years 5,8,11...; and a panel starting in year 3, covering years 6, 9, 12... provides a much larger sample size than an allocation of the same sites each year. Furthermore, three visits to panels provides essentially the same power for detecting a trend in the mean as the design that visits all sites each year (Urquhart and Kincaid, 1999). The added benefit of using a multi- panel design is that substantially more different sites are sampled than with the annual-alone design.
A basic structure for thinking about panels is in terms of whether the same set of sites will be revisited across years (i.e., the same sites every "x" years), or whether new sites will be visited across years (i.e., different sites every "x" years). The following illustrates several kinds of panel designs; see McDonald (2003) for a thorough treatment of panel designs. In the illustrations each row is a unique set of sites (a panel). The five examples shown below illustrate five temporal designs.
