Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Chapter 10 Analysis of Quantitative Data 1

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Vocabulary of Coding Coding - the systemic reorganization of raw data into a format that is computer readable. Coding Procedure - a set of rules stating that certain numbers are assigned to variable attributes. Codebook - a document describing the coding procedure and the location of data for variables in a format that computers can use. Precoding - the act of writing the code categories directly on the questionnaire; survey researchers often precode questionnaires before collecting data. 2

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Entering Data Data Field - a column or set of columns assigned to a variable. Code Sheets - a sheet with grids in which the researcher writes the code numbers in squares that correspond to a row and column location, and then uses this to type it into the computer. Direct Entry Method - a method of data entry where the researcher manually types in the data. Optical Scan Sheets - a machine that can read information from questionnaire sheets and insert them directly into a computer file. 3

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Cleaning Data Accuracy is extremely important when coding data. Errors made when coding or entering data into a computer threaten the validity of measures and cause misleading results. After careful coding, the researcher checks the accuracy of coding, or “cleans” the data. 4

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Results With One Variable Statistics - ways to manipulate and summarize numbers that represent data from a research project. Descriptive Statistics - a description of phenomena using numbers. Univariate Statistics - statistics describe one variable. Frequency Distribution - a table that shows the distribution of cases into categories of one variable, that is, the number or percent of cases in each category. Visually, this uses: 1.Histogram 2.Frequency Polygon 5

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Results With One Variable Measures of Central Tendency 1.Mode - the most common or frequently occurring number in a distribution of scores. 2.Median - the middle point of a distribution of scores (half the scores fall below the median and half the scores fall above the median). 3.Mean - the arithmetic average of a distribution of scores. 6

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Results With One Variable Measures of Variation Another characteristic of a distribution is its spread, dispersion or variability around the center. Researchers measure variation in three ways: 1.Range - the largest and smallest scores within a distribution of scores; the simplest way of measuring variation. 2.Percentile - indicate the score at a specific place within a distribution of scores. 3.Standard Deviation - a statistic based on the mean and gives an average distance between all scores and the mean. 7

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Results With Two Variables Bivariate Relationship - Considers two variables together and their relationship. Bivariate Statistics - statistics which describe the relationship between two variables. Statistical Relationship - when variables appear together and are associated (they co-vary). Statistical relationships are based on two ideas: 1.Covariation 2.Independence 8

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Results With Two Variables Bivariate Tables - The bivariate percentaged table presents the same information as a scattergram in tables that are based on cross tabulation and usually contain percentages. Bivariate Percentaged Table - a statistical table based on cross- tabulation; that is, the cases are organized in the table on the basis of two variables at the same time. Bivariate tables usually contain percentages. Cross Tabulation - the technique of intersecting two or more variables so that the cases are organized in the same table at the same time. 9

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Results With Two Variables Measure of Association - a single number that expresses the strength, and often the direction, of a relationship. Proportionate Reduction in Error (PRE) - the logic which underlies most measures of association. It asks the question: How much does knowledge of one variable reduce the errors that are made when guessing the values of the other variable? 10

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Inferential Statistics A genre of statistics that uses probability theory to test hypotheses formally, permit inferences from a sample to a population, and test whether descriptive results are likely to be due to random factors of to a real relationship. 11

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Statistical Significance Statistical Significance - the foundation of inferential statistics. It means that results are not likely to be due to chance factors. It indicates the probability of finding a relationship in the sample when there is none in the population. Level of Statistical Significance - a way of talking about the likelihood that statistical analysis results are due to chance factors (a relationship appears in the sample when there is none in the population). 12

Criminal Justice and Criminology Research Methods, Second Edition Kraska / Neuman © 2012 by Pearson Higher Education, Inc Upper Saddle River, New Jersey All Rights Reserved Type I and Type II Errors Type I Error Occurs when the researcher says that a relationship exists when in fact none exists. It means falsely rejecting a null hypothesis. Type II Error Occurs when a researcher says that relationship does not exist, when in fact it does. 13