Proportionator

The proportionator is the most efficient unbiased stereological method used to estimate population size in samples.

A typical application is counting the number of cells in an organ. The proportionator is related to the optical fractionator and physical dissector methods that also estimate population. The optical and physical fractionators use a sampling method called systematic uniform random sampling, or SURS. Unlike these two methods the proportionator introduces sampling with probability proportional to size, or PPS. With SURS all sampling sites are equal. With PPS sites are not sampled with the same probability. The reason for using PPS is to improve the efficiency of the estimation process.

Efficiency is the notion of how much is gained by a given amount of work. A more efficient method provides better results for the same amount of work. The proportionator provides a better estimate, that is a more precise estimate, than either of these two methods: the optical fractionator and physical dissector . The PPS is implemented by assigning a value to a sampling site. This value is the characteristic of the sampling site. The proportionator becomes the optical fractionator if the characteristic is constant, i.e. the same, for all sampling sites. If there is no difference between sampling sites, then the proportionator behaves the same as the optical fractionator. In actual sampling, the characteristic varies across the tissue being studied. Information about the distribution of the characteristic is used to refine the sampling. The greater the variance of the characteristic, the greater the efficiency of the proportionator. What this means to the stereologist is simple: if you need to count more and more to get the CE needed to publish just stop and switch to the proportionator.

The proportionator is a patented process that is not generally available. The only current licensee for the patent is Visiopharm.

The proportionator is the de facto standard method used to count cells in large projects. The increased efficiency provided by the proportionator makes more work intensive methods such as the optical fractionator less attractive except in small projects.

A common misconception in the stereological literature is that design based methodologies require that all objects of interest must have the same probability of being selected. It is true that making such a design decision ensures an unbiased result, but it is not necessary. The use of nonuniform sampling is often used in stereological work. The point sampled intercept method selects cells using a point probe. The result is a volume-weighted estimate of the size of the cells. This is not a biased result.

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