Descriptive Statistics & SPSS introduction Nan Yu COMM 420.8

Basic steps in conducting research Literature review Raise questions or make hypotheses Design of a study Collect evidence (qualitative evidence or quantitative evidence) Analyze your evidence (data) Draw conclusion from the analysis

Tasks for the next part of your group project Collect evidence Analyze your data Results and Conclusion Prepare for the presentation

Data Analysis Requirement Recommendation Understand statistics Be able to use statistical software Recommendation Take other stat classes or data analysis classes if you are truly interested in this area. This class will review very basic skills and principles about data analysis

A review – levels of measurement Nominal/Categorical e.g. Gender, Race, Favorite Music Genre Ordinal In a typical month, how many movies do you rent on video? None 1-2 3-5 5-20 More than 20

Strongly Disagree Neutral Strongly Agree Interval/Ratio I believe in love at first sight. Strongly Disagree Neutral Strongly Agree 1 2 3 4 5 On a typical day, how many hours of television do you watch?_______ How many siblings do you have?______

Text Book p.327, Table 14.1 Descriptive statistics --reduce and simplify the number to interpret the results Inferential statistics --make a judgment of what you observe in the sample can be generalized to the population from which the sample was drawn

Text Book p.327. Table 14.1 Nonparametric Parametric (continuous) Nominal, Ordinal Parametric (continuous) Interval, Ratio

Text Book p.327. Table 14.1 Univariate Bivariate Multivariate One variable Bivariate Two variables Multivariate Three or more variables Note: Sometimes scholars combine bivariate and multivariate so the distinction would be one variable (univariate) versus multiple variables (multivariate)

Descriptive Stats What to describe? What is the “center” of the data? How the data vary? (variability)

Measures of central tendency Mean Mode Median

Mean Another name of average If describing a population, denoted as μ, the Greek letter “mu.” If describing a sample, denoted as, called “x-bar.” “Balance point” or the “value center” of the distribution. Only use for interval/ratio.

Calculating the Mean Imagine that we have a population of 5 objects with heights of 2, 4, 6, 8, and 10 inches. X1 = 2 X2 = 4 X3 = 6 X4 = 8 X5 = 10 The mean of the population is: Sum of all the numbers divided by the number of observations contributing to that sum

Most frequently occurring score in a distribution. Mode Most frequently occurring score in a distribution. One data set can have many modes. Appropriate for all types of data, but most useful for categorical data.

Mode Hometown Number of students What is the mode in this case? Pennsylvania New York New Jersey Ohio Maryland Other States What is the mode in this case?

Median Middle score in a distribution (50% above, 50% below) Can use with ordinal and interval/ratio, but not with nominal

Median E.g., Scores: 4 9 2 2 1 8 10 9 7 Rank order them: 4 9 2 2 1 8 10 9 7 Rank order them: 1 2 2 4 7 8 9 9 10 Median

Summary Choice of descriptive measures depends entirely on the level of measurement for a particular variable.

A normal distribution Symmetric, bell-shaped curve. Most values fall around the mean, but some values are smaller, and some are larger. mean median mode

But distributions are not always normal (p.345) Skewness: How far the peak is from the center of the distribution

What happens if the distributions are not normal? Impact of skewness on measures of Central Tendency Mode Mode Median Median Mean Mean Right (Negative) Skew Left (Positive) Skew

Distribution Kurtosis (p.345) Normal (bell-shaped) peaked flat

Dispersion The mean, mode and median are not enough to understand the distribution of a variable.

Variability How variable are the scores in a distribution? Range Standard Deviation Variance

Range

Standard Deviation A measurement of variability that indicates how much all of the scores in the distribution typically deviate from the mean.

Calculating Standard Deviation

Variance

What is the standard deviation for this distribution?

SPSS Statistical Package for the Social Science A list of other available statistical software, p.332 Please download the file “Class Survey” from ANGEL (week 9) and save it on your desktop

Introduction to SPSS Opening a data file STEP 1 Double click on the “class survey.sav” file. STEP 2 Select "Edit," then "Options" STEP 3 Select "Display names" and "File," then Click "OK"

Introduction to SPSS Variables Cases (People)

Variable Labels To See Value Labels Place cursor over variable name Click Value Label button

Sorting Cases Click Data -> Sort Cases Select Variable, Move to Right Box, Click on OK when ready.

Data Editor in Variable View Information About Each Variable Across Variables Listed Down Values

Frequencies/Descriptive Statistics Requesting frequencies Analyze -> Descriptive Statistics -> Frequencies

Select the variables you want to analyze, and use the arrow keys to move them into the "Variables" box. Click "Statistics." Select the statistics you want, and then click "Continue" and then "OK."

Output Window You can double-click on the chart and edit the appearance… You can copy and paste the chart into other applications... Notice the new window...

Graphs A. Select "Graphs" then "Histogram" B. Put “tvhours" in the variable box. C. Select "Display normal curve" D. Select "OK"

In Your Output Window You can double-click on the graph and edit the appearance. You can copy and paste the graph into other applications.

In Your Output Window As new procedures are run, they are appended to the output and added to the outline on the left of the screen. You can navigate around your output by selecting from the outline menu. You can also delete any output by selecting it and then clicking the delete key.

Entering Your Own Data A. Open Data (Type in Data)

Label for the Variables Variable Names Listed Down B. Go to “Variable View” and name and label variables Label for the Variables Variable Names Listed Down (No spaces) Labels for the Values Values Dialog Box: Click “Add” after each new value label that you include.

C. Enter data in “Data View” Cell Editor Case Cell 1. Click on a given cell where you want to add data. 2. Type the value you want to include in that cell. The value will appear in the cell editor. 3. The TAB key will take you to the next variable in the same case. 4. The ENTER (return) key will take you to the next case.

D. Save your data after each case I lost all my data because I forgot to save it. Oh-my-gosh! I think I forgot to save the data! Ha ha ha. Too bad for them. My data is safe!

Saving Your Data and Output Extensions Used for File Types Date Files: .sav e.g., "mydata.sav" Output Files: .spo e.g., "myoutput.spo"

Saving Your Data File A. From within the Data Editor, select "File" then "Save as" B. Select your floppy disk. C. Type in the name of your data file (with the extension .sav). D. Select "Save." Note: After you have saved your data for the first time, you can save it periodically by selecting "Save" rather than "Save as."

Saving Your Output File A. From within the Output window, select "File" then "Save as" B. Select your floppy disk. C. Type in the name of your output (with the extension .spo). D. Select "Save." Note: After you have saved your output for the first time, you can save it periodically by selecting "Save" rather than "Save as."

Z-scores If two measures were used to measure the liking toward “Nan Yu” not likable 1 2 3 4 5 likable not nice 1 2 3 4 5 6 7 nice The raw scores are not comparable because they are measured differently.

So we need to standardize the scores… What we do is… Translating each individual score. The transformed scores will necessarily have a mean of zero and a standard deviation of one. The standard score indicates how many standard deviations an observation is above or below the mean.

-3 -2 -1 1 2 3

Requesting Z-scores from SPSS Open Class Survey.sav Go to Analyze  Descriptive Statistics  Descriptive

Z-scores in SPSS A. Put “tvpm" in the variable box. B. Check “Save standardize values as variables, can click OK

Z-scores in SPSS

Practice Request z-scores for “tvsit.” Sort “Ztvsit” in a descending order.

In-Class Demo Run frequencies on “wwwbuy” Request mean, median, mode, standard deviation, range, and skewness, and historgram with normal curve. Does it look at a normal distribution? Base on the reported mean, median and mode, explain the skewness of the distribution of “wwwbuy.”

In-class Demo answer You can compare your SPSS output with the one on ANGEL (class demo answer.spo) No, it doesn’t look at a normal curve. The mean is higher the median and the mode. So it is a positive (left) skew.