Читать книгу Experimental Design and Statistical Analysis for Pharmacology and the Biomedical Sciences - Paul J. Mitchell - Страница 26

Consider a simple experiment where you are trying to assess the effect of a drug or serum on the population growth of cells in culture. You carefully prepare two flasks containing equal volumes of identical growth medium, except that one flask also contains the drug or serum you are interested in. From your stock flask of cells in culture, you carefully remove an exact volume and add the aliquot to one of the flasks. As carefully as possible you repeat the process and add another aliquot to the second flask. The assumption here is that you have added the same number of cells to each flask so that the cell concentration in each flask is the same. You then leave the flasks to incubate. After a suitable incubation period (e.g. three days) you take four samples from each flask and estimate the cell concentration using a haemocytometer. This process is summarised in Figure 3.2.

Figure 3.2 Measurement of cell population in quadruplicate.

In this example, the cell population in each flask is measured in quadruplicate. This is a process whereby the precision of the final population value is improved by taking more than just one reading. It is important to note, however, that these are not independent readings as in each case the four samples are taken at the same time from the same flask. Consequently, the four values are averaged to provide a single value which is the best estimate of the population of cells in each flask, i.e. n = 1 for each flask. Such a process has often been erroneously misinterpreted as providing an n = 4, but this is incorrect simply because the four values for each flask are not independent. [See also Chapter 20, Example 20.3.]