Copyright 2017 Elsevier Inc. All rights reserved. 291 Calculating Descriptive Statistics There are two major classes of statistics: descriptive statistics and
inferential statistics. Descriptive statistics are computed to reveal characteristics of the sample data set and to describe study variables. Inferential statistics
are computed to gain information about effects and associations in the population being studied. For some types of studies descriptive statistics will be the only
approach to analysis of the data. For other studies descriptive statistics are the rst step in the data analysis process to be followed by infer-ential statistics.
For all studies that involve numerical data descriptive statistics are crucial in understanding the fundamental properties of the variables being studied. Exer-cise
27 focuses only on descriptive statistics and will illustrate the most common descrip-tive statistics computed in nursing research and provide examples using actual
clinical data from empirical publications. MEASURES OF CENTRAL TENDENCY A measure of central tendency is a statistic that represents the center or middle of a
frequency distribution. The three measures of central tendency commonly used in nursing research are the mode median ( MD ) and mean ( X ). The mean is the
arithmetic average of all of a variable s values. The median is the exact middle value (or the average of the middle two values if there is an even number of
observations). The mode is the most commonly occurring value or values (see Exercise 8 ). The following data have been collected from veterans with rheumatoid
arthritis ( Tran Hooker Cipher &Reimold 2009 ). The values in Table 27-1 were extracted from a larger sample of veterans who had a history of biologic medication
use (e.g. iniximab [Remi-cade] etanercept [Enbrel]). Table 27-1 contains data collected from 10 veterans who had stopped taking biologic medications and the
variable represents the number of years that each veteran had taken the medication before stopping. Because the number of study subjects represented below is 10 the
correct statistical notation to reect that number is: n=10 Note that the n is lowercase because we are referring to a sample of veterans. If the data being presented
represented the entire population of veterans the correct notation is the uppercase N. Because most nursing research is conducted using samples not popu-lations all
formulas in the subsequent exercises will incorporate the sample notation n. Mode The mode is the numerical value or score that occurs with the greatest frequency; it
does not necessarily indicate the center of the data set. The data in Table 27-1 contain two
EXERCISE 27 292EXERCISE 27 Calculating Descriptive StatisticsCopyright 2017 Elsevier Inc. All rights reserved. modes: 1.5 and 3.0. Each of these numbers occurred
twice in the data set. When two modes exist the data set is referred to as bimodal ; a data set that contains more than two modes would be multimodal . Median The