Module Components

• Overview

Topics

• Describe Associations: Cross Tabulation/
  Contingency/Pivot Tables
• Statistically Testing Association: Chi-Square
• Measuring Disease
• Simple Logistic Regression

NRSG 795: BIOSTATISTICS FOR EVIDENCE-BASED PRACTICE

Module 8: Associations Between Nominal or Ordinal Variables

Describe Associations: Cross Tabulation/Contingency/Pivot Tables

Cross tabulation is one of the most useful analytical tools and is a main-stay of the research analysis. One esti­mate is that single variable frequency analysis and cross-tabulation analysis account for more than 90% of all research analyses.

The cross tabulation/contingency table is the categorical equivalent of the scatterplot. In a scatterplot, there are usually many values for a variable (e.g. weight of heart failure patients, systolic BP) since the variables are interval or ordinal level measures. A cross tabulation is a two (or more) dimensional table (displayed in matrix/grid form) that records the number (frequency) of respondents that have the specific characteristics described in the cells of the table. Cross tabulation tables provide a wealth of information about the relationship between the variables. Not only can it be used to provide count information, but it is also useful in describing percentages. A contingency table allows you to better understand the data using probability and relative frequencies (visual). For example by placing the data in a table (see below), some conclusions can be drawn that might not be evident if the data was just in a list. The user can see that there seems to be a correlation between the length of a cold and taking medicine.

Excel uses a pivot table as a data summarization tool (found in data visualization programs such as spreadsheets or business intelligence software). Among other functions, a pivot table can automatically sort, count total or give the average of the data stored in one table or spreadsheet, displaying the results in a second table showing the summarized data. Pivot tables are also useful for quickly creating unweighted cross tabulations. The user sets up and changes the summary's structure by dragging and dropping fields graphically. This "rotation" or pivoting of the summary table gives the concept its name.

Required Readings and Videos

• Understanding Contingency Tables and Crosstabulation, Pt. 1 (21:18) https://www.youtube.com/watch?v=Rwc4-MXl8VI

Learning Activity

• Complete the learning activity here and check answers.