Evidence-Based Practice for Nurses

Causality and Probability

Causality refers to the relationship that exists between a cause and its effect.
The cause has the ability or power to produce a specific effect. The assumption
is made that the cause precedes the effect. Being able to determine causality
requires that the variables of interest are studied in chronological order: the
causal variable, or IV, has to occur prior to the occurrence of the DV, or the
effect. From a research perspective, causality is the belief that researchers, us-
ing stringent controls, can manipulate a cause to produce the effect or result
that can be repeated. Experimental designs are used to examine causality, with
researchers manipulating the IV while observing its effect on the DV. When
working with people, nurse researchers recognize that there are multiple factors
that can contribute to an outcome. Rarely do nurses deal with problems that
have only one cause and one effect. When outcomes have many causes, it is
known as multicausality. Therefore, it is important to consider multicausality
when evaluating research designs. Because it is possible that there are multiple,
interrelated factors causing an outcome, it is important for researchers to try
to identify and control for as many of those factors as possible. When apprais-
ing research studies, nurses need to familiarize themselves with the variables
included in the study and evaluate the discussion section of the study report
to determine whether researchers identified other possible causes for the
results. Box 6-1 outlines points that should be considered when appraising
quantitative designs.

Closely related to the idea of multicausality is the idea of probability. Again,
because nursing is a human science rather than a basic science, such as biology
or chemistry, nurse researchers are unlikely to be able to assert with absolute
certainty that the IV caused the effect in the DV. Instead, research related to
human health and functioning results in assertions of probability or how likely
it is that the change in the DV was caused by the IV. Probability assertions

KEY TERMS

causality: The

relationship

between a cause

and its effect

probability:

Likelihood or

chance that an

event will occur in a

situation

What type of quantitative design is being used?

Can causality be inferred?

Did researchers use the highest level of design possible to answer the research questions?

What strategies did the researcher use to control for the effects of extraneous variables?

What threats to internal and external validity might be present, and were any strategies used to

reduce them?

Were there any ethical concerns about the design?

BOX 6-1

Elements to Consider When Appraising Quantitative

Designs

6.1 Chart the Course: Selecting the Best Design 151