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Statistics Review _2 (Long - Astrology) - Student Version

# Statistics Review _2 (Long - Astrology) - Student Version -...

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PSY 0035L: Research Methods Lab Name:______________________________________ I N -C LASS E XERCISE #3 S TATISTICS R EVIEW O BJECTIVES The purpose of these exercises is to review various statistical tests that will important as you begin to design your own experiments in the next couple of weeks. This review covers: the difference between descriptive and inferential statistics median, mean ( M) , mode, and standard deviation ( SD ) the importance of statistical significance and its relationship to probability the t- test the chi square 2 ) statistic As part of the exercises you will be asked to calculate by hand statistics such as M, median , mode, t, and χ 2 . You will also be introduced to the appropriate way of reporting statistical results in APA style. D ESCRIPTIVE AND I NFERENTIAL S TATISTICS Descriptive statistics summarize data. For example, suppose you have the scores on a standardized test for 500 subjects. One way to summarize the data is to calculate a measure of central tendency ( mean, median , or mode ) which indicates how the typical person scored. You might also determine the highest and lowest scores, and the spread of a distribution which would indicate how much the scores varied (range and standard deviation). Inferential statistics are tools that tell us how much confidence we can have when we generalize from a sample to a population. You are familiar with national opinion polls in which a carefully drawn sample of only about 1,500 adults is used to estimate the opinions of the entire adult population of the United States. The pollster first calculates descriptive statistics, such as the percentage of respondents who are in favor of capital punishment and the percentage who are opposed. Having sampled, he or she knows that the results may not be accurate because the sample may not be representative; in fact the pollster knows that there is a high probability that the results are off by at least a small amount. This is why pollsters often mention a margin of error , which is an inferential statistic. It is reported as a warning to the audience that random sampling may have produced errors, which should be considered when interpreting results. For example, a weekly news magazine reported that in a national poll 58% of the respondents believed that the economy was improving; a footnote indicated that the margin of error was ±2.3. This means that the pollster was confident that the true percentage for the whole population was within 2.3 percentage points of 58%. You probably recall that a population is any group in which a researcher is interested. It may be large, such as all adults age 18 and over who reside in the United States, or it might be small, such as all registered nurses employed by a specific hospital. Researchers are free to choose populations of interest and should clearly define them when writing reports of their studies. A study in which all members of a population are included is called a census. A census is often feasible and desirable when working with small populations (e.g., an algebra teacher may wish to pretest all students at the Page 1 of 17 Revised 9/30/09

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