What is Design-based Stereology
The term “design-based” is used to describe newer methods in stereology whose probes and sampling schemes are ‘designed’, that is, defined a priori, so that the methods are independent of the size, shape, spatial orientation, and spatial distribution of the geometrical features to be investigated. Rules must be followed to achieve this independence, and if they are followed correctly than bias is eliminated. Older stereological methods were “model-based”, this means that they used models based on the geometric properties of the objects being studied. The design-based methods eliminate the need for using information about the geometry of the objects to be investigated, resulting in more robust data because potential sources of systematic errors in the calculations are eliminated.
Design-based stereology considers solutions in which models are avoided. The design includes making sure the combination of the tissue collection (e.g. sectioning method) and the nature of and orientation of the virtual geometric shapes used to probe the tissue obviate the need for knowledge about the geometrical features being studied to make a model. There are four basic assumptions that are avoided by design-based approaches:
- No assumptions about shape
- No assumptions about size
- No assumptions about orientation
- No assumptions about distribution
Design-based stereology can be divided into two main components: 1) analyses of the global and 2) local characteristics of tissues. Global characteristics deal with regions. These characteristics can be expressed as absolute values (e.g., the number of granule cells in the rat cerebellum or the volume of the cortex) or as relative values (e.g., the volume fraction of the rat cerebellum occupied by the granule cell layer, the density of granule cells within the granule cell layer of the rat cerebellum). Local characteristics are concerned with particles (e.g. the volume and/or surface area of a neuron). Both global and local characteristics can be analyzed by a variety of design-based probes. Many local probes can be used during global probes. For example, while using the optical fractionator probe to estimate the number of cells in a region, the nucleator probe can also be employed to estimate the volume of the cells.
Why not use model based stereology?
Model based stereological methods use approximation methods to describe objects which are being studied. These approximations work only as well as the models truly represent the actual objects. The problems with models can be avoided by simply avoiding the use of models.