tailieunhanh - Ebook Handbook of biolological statistics (3/E): Part 2

(BQ) Bart 2 book “Handbook of biolological statistics” has contents: Nested anova, multiple comparisons, multiple logistic regression, simple logistic regression, multiple regression, curvilinear regression, using spreadsheets for statistics, choosing a statistical test, and other contents. | HANDBOOK OF BIOLOGICAL STATISTICS Normality Most tests for measurement variables assume that data are normally distributed (fit a bell-shaped curve). Here I explain how to check this and what to do if the data aren’t normal. Introduction Histogram of dry weights of the amphipod crustacean Platorchestia platensis. A probability distribution specifies the probability of getting an observation in a particular range of values; the normal distribution is the familiar bell-shaped curve, with a high probability of getting an observation near the middle and lower probabilities as you get further from the middle. A normal distribution can be completely described by just two numbers, or parameters, the mean and the standard deviation; all normal distributions with the same mean and same standard deviation will be exactly the same shape. One of the assumptions of an anova and other tests for measurement variables is that the data fit the normal probability distribution. Because these tests assume that the data can be described by two parameters, the mean and standard deviation, they are called parametric tests. When you plot a frequency histogram of measurement data, the frequencies should approximate the bell-shaped normal distribution. For example, the figure shown at the right is a histogram of dry weights of newly hatched amphipods (Platorchestia platensis), data I tediously collected for my . research. It fits the normal distribution pretty well. Many biological variables fit the normal distribution quite well. This is a result of the central limit theorem, which says that when you take a large number of random numbers, the means of those numbers are approximately normally distributed. If you think of a variable like weight as resulting from the effects of a bunch of other variables averaged together—age, nutrition, disease exposure, the genotype of several genes, etc.—it’s not surprising that it would be normally distributed. 134 .