BIOL2300_Chapter%202 - CHAPTER 2 Description of Populations...

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16 CHAPTER 2: Description of Populations and Samples This lecture topic concerns chapter 2 in your textbook, which is about how statisticians describe populations and samples. We’ll spend a lot of time discussing how to describe samples, and only at the end will we really start to talk about populations. We’re going to take a look at the following topics I. Variables and data (the numbers we work with) II. Frequency distributions III. Descriptive statistics referring to what’s called the center of the data IV. Graphs called box plots V. Descriptive statistics referring to what’s called the dispersion of the data VI. Transformations, meaning mathematical manipulations of the data VII. Samples, populations, and something called statistical inference I. Variables – the raw material for statistics These are the things we measure or observe on whatever we are studying. The particular observations or measurements we get vary from individual to individual, so if we have a collection of data from several individuals, we get a bunch of different values. The goal of statistical analysis is to try to make sense of this variation among individuals. Before looking at any procedures for statistical analysis, a basic point to understand is that there are different kinds of variables, and each type has particular implications for how we might do statistical analysis. One type of variable is a quantitative variable , meaning one that is expressed numerically. There are two different kinds of quantitative variables, distinguished by the kinds of numerical values they take. Continuous variables – can take any decimal or fractional value: 1.43 2.5978496 0.666666… Things like weights, heights, concentrations and so on are naturally measured and expressed this way.
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17 Discrete variables – take only integer values: 1, 2, 3, … -2, -1, 0, 1, 2, … Discrete variables usually arise when we count something: number of teeth, number of eggs, … Another kind of variable does not have numbers associated with it at all – this kind of variable is called a categorical variable . This refers to characteristics of things that can be classified into different categories: Male, Female Dead, Alive Genotype – AA, Aa, aa Because statistics deals with numbers, we have to associate quantities with nominal variables. We do this with frequencies or proportions . The term frequency refers to how many individuals fall in the different categories. For example, we could look at the frequency of males and females in this room, but counting up the numbers of each. We could also divide those frequencies by the total number of people, and then we would have the proportions of men and women in the room. Another type of variable has an in-between status – ordinal variables .
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This note was uploaded on 01/27/2010 for the course BIOL 2300 taught by Professor Britton during the Spring '08 term at UT Arlington.

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BIOL2300_Chapter%202 - CHAPTER 2 Description of Populations...

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