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First, the gold standard for viability determination is the trypan blue exclusion assay where viable cells exclude the dye and dying and dead cells do not. This is a short-term assay and easy to do. Many automated viability assays use trypan blue as the dye but then just automatically calculate viability based on some algorithm. The advantage to the automated system is that they tend to count many more cells and counting squares than done when counting manually. This means the statistical probability is greater, meaning more accuracy in the count. Another nice feature about automation is better documentation and the ability for validation. The drawback to automation is that one might stop looking at the cells under a microscope, and I think it is always a good idea to see the cells under a microscope. Personally, if I were evaluating I would compare directly a manual count against a automated instrument. I would compare multiple cell types and media and between instrument I would compare ease of use, upkeep and cost of use. So you know, other methods are available for mammalian cell viability assays such as fluorescent assays developed by Molecular Probes (http://www.lifetechnologies.com/us/en/home/life-science/cell-analysis/cell-viability-and-regulation/cell-viability.html).