Evidence-Based Practice for Nurses

Determining Sample Size

After the sampling method is identified, researchers must decide how large a
sample is needed. Various factors determine sample sizes for quantitative and
qualitative studies. In either type of design, the sample size must be sufficient to
adequately support the research purpose and design of the study. Homogeneity
of the population is one factor that needs to be considered when determining
sample size in quantitative studies. Homogeneity refers to the degree to which
elements are homogenous or similar (Bloom & Trice, 2011). If a population
has a number of common characteristics, the sample size does not need to be
as large as a more heterogeneous, or diverse, population (see Figure 11-3).

Another factor to consider when determining sample size is the purpose
of the study and what is being studied. If the focus of the study is narrow, the
sample may not need to be as large as a sample for a very broadly focused
study. A study investigating eating habits of fifth graders is very broad and
would require a large sample. A study on eating habits of fifth graders who
have diabetes is narrower in focus; thus, the number of subjects needed for
the study may be smaller.

11.3 Sample Size: Does It Matter?

At the end of this section, you will be able to:

‹ Discuss factors that should be considered when determining sample size

‹ Describe strategies that enhance the recruitment and retention of subjects

‹ Identify factors related to sampling that must be considered when appraising studies

for evidence-based practice

XOXO

XOXO

XO

XO

XXX

X

XXX

O

Homogeneous

sample

Heterogeneous

sample

FIGURE 11-3

Homogeneous vs. Heterogeneous

Samples

KEY TERMS

homogeneity: The

degree to which

elements are similar

or homogenous

heterogeneous:

The degree to

which elements are

diverse or not alike

11.3 Sample Size: Does It Matter? 299