Descriptive Statistics

Transcription

1 Descriptive Statistics Available on the class website The rubric is also posted if you want to see how they will be graded You ll need some of the output from today to complete the assignment Let s take a look at what you ll be writing about 1

2 Let s log in Use your student log in information that you use for any campus computer and probably your campus Get out the lab handouts for today we ll walk through an example from there It also contains practice problems that you can complete SPSS is a program that allows us to run many of the analyses used by psychologists Why do you need to know how to use it? Expected of psychology majors If you re going to graduate school, you ll need it for theses If you re not going to graduate school, employers will expect that you are familiar with how to understand reports of data and possibly even collect and use data. It can only help your career prospects. 2

3 Descriptive statistics are values that describe the characteristics of a sample or population. The first type is measures of central tendency (the mean, median, and mode), which are statistics that describe the typical value in a sample or population. The second type is measures of variability (the range, standard deviation, and variance), which are statistics that describe how different the scores in a sample or population are from each other. You should use descriptive statistics when you wish to describe the average value and/or the amount of variability in a sample or population. 3

4 What is the typical value in a set of scores? How variable are a set of scores? What is the average household income of children diagnosed with Attention Deficit Hyperactivity Disorder? Do students at Ivy League universities all have the same high school grades and SAT scores (e.g., everyone has a 4.0 and a 1600) or is there a large amount of variability in the students grades and SAT scores (such that some students have very high scores and others very low scores)? Stella has noticed that Aggies are always saying howdy to her. She wonders if Aggies are just more extraverted than your average person. So, Stella gives 20 Aggies a questionnaire called the Extraversion IQ Instrument that provides persons with an extraversion score. Like a normal IQ measure, the Extraversion IQ Instrument is scaled such that a score of 100 means you have an average amount of extraversion. Stella wants to know (a) if Aggies are more extraverted than normal and (b) if all Aggies are extraverted or if Aggies are variable in extraversion. 4

5 Stella wants to know if Aggies are more extraverted than normal. To address this question she is using descriptive statistics. The problem with relying solely on descriptive statistics ti ti to draw inferences such as more extraverted t than normal is that the meaning of more than normal is vague. How much higher than 100 would the Aggie sample mean have to be for Stella to declare it larger than 100? There are statistics called inferential statistics that can be used to quantify exactly how large something needs to be to call it significant (to be covered soon). For now,,just use your subjective judgment to determine whether an average is larger than another average or whether a sample has a large or small amount of variability. However, you should know that this is, at best, a quick and dirty way to do this, and that inferential statistics are much more appropriate. Because Stella is interested in describing Aggies average amount of extraversion (a continuous measure), the mean, median, and mode are each appropriate descriptive statistics. Additionally, because Stella is interested in describing the degree to which Aggies vary in extraversion, the range, standard deviation, and variance are each appropriate statistics (although you don t normally report the variance because it s difficult to interpret squared units such as amount of extraversion squared). 5

6 Open dataset: Chapter 2 Example 1.sav The sample of 20 Aggies was average to slightly-above-average in Extraversion IQ scores (M = ). There was also substantial variability between Aggies in extraversion scores (SD = 15.33), with Extraversion IQs ranging from 73 to 130 (higher Extraversion IQs mean more extraversion). **Why did we report the mean here? Why not median or mode? 6

7 Aggies are about average in extraversion (maybe slightly above average). Some Aggies are very introverted, some are very extraverted, and all points in between. Less interpretation is needed for participant demographics (reported in the method) Participants were 18 college students with a mean age of 20 years (SD = 1.2) and 52% were female. 7